Start here: what your symptom tells you
Generator runs, nothing electrical works at all.
Start at the breaker on the generator itself, then the transfer switch. If the 120 volt side is dead everywhere, the charger never sees any input and the batteries cannot charge.
Outlets work on the generator, batteries still flat.
Very useful. The generator, its breaker and the transfer switch are all working. Your fault is now in the same place a shore power charging fault would be: the charger, the disconnect, the inline fuse near the battery, or the battery.
Everything works but the generator labours or bogs.
The load is too big for it. A battery charger can pull up to 3,000 watts and arrives uninvited, so adding it to air conditioning can tip a small generator over.
Charger barely runs, or refuses.
If you have a newer inverter charger, check its AC input current limit. Manufacturers publish settings for exactly this and describe the symptom as the charger hardly charging or not charging at all.
Portable generator, plugged in with a 12 volt cable.
Stop, and read the section on that below. It does not work the way people expect.
The generator is not the charger
This is the fact the whole guide rests on, and the manufacturer states it directly. Asked whether a generator can charge an RV house battery, the answer given in Cummins Onan's own handbook is that it cannot, not directly, but that a generator provides power to operate your converter or charger, which charges the batteries. The same answer adds one caveat worth knowing, that many diesel generators also carry an alternator which can charge batteries directly, so a diesel RV has a second charging path that a gas one usually does not.
That single sentence explains a lot. It means the generator's job ends at producing 120 volts AC. It means the thing doing the charging is the same device that charges your batteries on shore power. And it means a generator that runs perfectly can be entirely innocent while your batteries sit flat.
Worth knowing, because it is unusual: Onan's generator documentation does not contain a house battery charging failure table. We looked, across installation, operator and service material. That is not an oversight. House charging is the coach's system rather than the generator's, so the manufacturer treats it as out of scope, and the guide you actually need is the converter one.
And the manufacturer's own order of checks is worth following exactly, because it is written by people who see these failures daily. Asked what to do when there is no AC at any outlet with the generator running, their answer begins with the circuit breakers at the generator being off or tripped, and says to check those first. Then it adds that if the breaker is on, there is AC at the generator, and there is still nothing at the outlets, the problem could be a faulty transfer relay, and notes specifically that this device is located in the coach and is not part of the generator.
The ranked causes
Documented by manufacturers:
One, the generator's own breaker is off or tripped.
On a 50 amp coach there may be a pair of them. This is the manufacturer's first check and it costs nothing.
Two, a failed transfer switch.
Stuck in the shore position, or with burnt contacts, it never lets generator power through. Transfer switch makers publish their own test procedures, and the part is far cheaper than a generator repair.
Three, the charger itself failed or is current limited.
An inverter charger engages its charging function automatically once it senses a qualified AC input, and the practical test is simply whether battery voltage climbs after it does. Where the inverter works but the charger does not, Xantrex's guidance is to check the AC input voltage at the inverter and confirm it is within the unit's range.
Four, the battery disconnect or the inline fuse near the battery.
Exactly the same break points as a shore power fault, because it is the same wire. If the disconnect is open, no charger can help.
Five, an inverter charger's AC input limit set wrong for the generator.
Victron publishes a starting point of 80 percent of the generator's capacity in watts. They also publish a setting called WeakAC, described as being for when the charger is hardly charging or not charging at all, which reduces maximum charge current by approximately 20 percent. This is a settings problem that looks exactly like a hardware failure.
Six, an undersized generator.
Covered in the sizing section below, and more common than people expect once air conditioning is in the picture.
Field evidence without manufacturer documentation.
A fault in the wiring between the generator and the transfer switch, including a loose junction box connection, is repeatedly described by owners and technicians. One account notes that a coach has two hot legs in that run, and if the one feeding the converter is dead then the batteries will not charge even though the generator runs. Generator output faults such as a corroded brush block or slip rings, giving zero output at the terminals, are also field-reported. Both are plausible and neither appears in a manufacturer failure table we could find, so treat them as things to test after the documented causes are eliminated.
Sizing: can it run the charger and the air conditioner?
This catches people out because the charger is invisible. It switches on when it wants to, and it is not small.
The manufacturer's own numbers.
A battery charger is listed as drawing up to 3,000 watts, or roughly 6 to 28 amps, and the documentation notes that chargers come on automatically and can draw a large load. A converter is smaller at 500 to 1,000 watts, about 4 to 8 amps. An air conditioner running draws 1,200 to 2,400 watts, and its start-up surge is three to four times that. For scale, a 30 amp shore connection delivers about 3,600 watts and a 50 amp connection about 12,000.
Manufacturer recommended generator sizes by RV type, which is a useful sanity check on your own setup: a Class B with one air conditioner, 2,500 to 2,800 watts. A Class C with one, 3,200 to 4,000. A Class A with two 15,000 BTU units, 5,500 to 8,000. A Class A with three, 10,000 to 12,500. A fifth wheel with two, 5,500 to 7,000.
How to use this.
Add the running watts of everything you want on, add the start-up surge of the largest appliance, add the charger's draw, and compare against the generator and the RV's own breaker. If the total is over, either manage the loads or slow the charging down. Many inverter chargers let you reduce charge current precisely so the generator can cope, which is a settings change rather than a repair.
Do not use the generator's 12 volt outlet
This is a specific trap with portable generators, and it is worth understanding rather than just avoiding.
That outlet is not a charger.
The trade guidance from a named expert is blunt: the 12 volt DC outlet is not voltage or current regulated, so it will not properly charge RV batteries directly without an external charge controller. It is limited to a maximum of about eight or ten amps, and in practice usually nearer five, which works out to around 60 watts of charging power. The same generator's 120 volt outlet, feeding your existing 40 or 80 amp charger through the shore cord, delivers somewhere between 1,000 and 1,600 watts.
What to do instead.
Plug the generator into the RV's shore power inlet with the appropriate adapter and let the onboard charger do the work. That is how the system is designed to be fed, and it is why the onboard charging path matters even when you are boondocking.
The same is true of an onboard generator's own DC circuit, which is typically a much smaller regulated output, and not the path used to charge the house bank.
What it costs
Again, no manufacturer publishes repair pricing, so these are commercial sources and ranges.
A transfer switch is a modest part.
A common 50 amp unit runs roughly $189 to $350, with heavier 100 amp units around $695 to $798. We could not find an RV specific installed price from any attributable source, only part prices, so treat labour as an unknown and ask your shop for it directly.
Generator repair is where the money is, quoted broadly from $150 up to several thousand, with most owners spending somewhere between $350 and $900. A mobile service visit with oil, filter and plug runs around $280 to $350, and a full service suite closer to $485. Authorised generator service labour is quoted at roughly $175 to $225 an hour, with independent shops nearer $145 to $175.
The comparison worth making.
A generator service visit and a transfer switch replacement are in the same broad price band. Because the manufacturer's own priority list puts the breaker first and the transfer switch second, and because both are cheaper than generator work, insisting on those two checks before authorising generator repair is the sensible order.
The rest of this cluster
Because the generator's charging path runs through the charger, the converter not charging guide covers the second half of this fault and is worth reading next. The 12 volt diagnostic guide has the whole system and the voltage-drop method. If a fuse is blowing rather than a battery not charging, that is fuse keeps blowing.
Why won't my generator charge my RV batteries?
Because a generator never charges them directly. It makes 120 volt AC, and that AC has to pass through a transfer switch to reach the converter or inverter charger, which is the thing that actually charges the batteries. So when a generator runs and the batteries stay flat, the fault is almost always downstream of the generator, in the transfer switch, the charger, or the same fuses and disconnect that a shore power fault would involve.
What is the first thing to check when the generator runs but nothing has power?
The circuit breaker on the generator itself. It is the first thing the manufacturer's own troubleshooting guide tells you to check, ahead of anything else. They also note that if the breaker is on and there is still no power at the outlets, the likely cause is a faulty transfer relay, and that relay lives in the coach rather than being part of the generator.
Why do my outlets work on the generator but the batteries still don't charge?
That is useful information, because it means the generator and the transfer switch are both working and the fault is further along. It narrows to the charger itself, the battery disconnect, the inline fuse near the battery, or the battery. It is the same set of break points a shore power charging fault goes through.
Can I charge my RV batteries from a portable generator's 12 volt outlet?
You should not rely on it. That outlet is not voltage or current regulated, so it cannot properly charge a battery without an external charge controller, and its output is tiny. The documented figure is a maximum of about eight or ten amps but usually around five, which is roughly sixty watts of charging power, against the thousand to sixteen hundred watts available from the same generator's 120 volt outlet. Use the 120 volt output into the shore inlet instead.
How do I know if my generator is too small to run the air conditioner and charge the batteries at once?
Add up the loads. A battery charger can draw up to 3000 watts and comes on automatically, while a 15,000 BTU air conditioner draws about 1,500 watts running and up to 3,500 watts starting. Those two alone can exceed a small generator, which is why manufacturers publish recommended wattages by RV type, from around 2,500 watts for a Class B with one air conditioner up to 10,000 to 12,500 for a Class A with three.
Why does an inverter charger sometimes refuse to charge from a generator?
Because many units limit how much AC current they draw in order to protect the generator, and if that limit is set wrong the charger barely runs or does not run at all. Victron suggests starting at 80 percent of the generator's capacity in watts, and lower on a 3000 rpm generator than a 1500 rpm one. They also publish a WeakAC setting for exactly this symptom, described as being for when the charger is hardly charging or not charging at all.
Sources
The manufacturer and agency documents this guide draws on, so you can check any figure for yourself.