RV black tank routines: How to dump, treat, store, and diagnose

Start here: The gauge that reads full on an empty tank

This is the complaint that sends most people looking for parts, and it usually is not a broken part. Most factory tank sensors are conductivity probes mounted through the tank wall. When liquid touches a probe it completes a circuit and the panel lights that level. Waste, toilet paper fiber or soap film left on a probe completes the same circuit with the tank nearly empty, which is why a panel that reads a third or two thirds on a freshly dumped tank is almost always reporting residue rather than depth.

So the first move is a clean, not a purchase: fill the tank partway with water, add a treatment that targets paper, let it sit for the time the label gives, drive if you can so the motion works the residue loose, and dump only when the tank is nearly full. Using the black tank flush while dumping helps as well. If the reading survives that, the tank sensor guide separates a fouled sensor from a wiring fault with two tests, The classic fault has a signature: a panel that always reads two thirds usually has residue or paper fibers bridging the two thirds probe, and there is usually no electronic reset, just cleaning. And you can look for yourself: press the toilet foot pedal down fully and shine a flashlight through the waste gate opening.

How to dump a black tank

Keep the black tank valve closed, and never leave it open. Let the tank fill to nearly full before dumping, because the volume of liquid behind the waste is what carries the solids out. Emptying a part-full tank moves the liquid and leaves the solids. Use enough water with every flush that the waste keeps moving rather than settling on the tank floor. When you do dump, it is black first and gray second, so the gray water rinses the outlet and the hose behind the black water.

The valve should always stay closed at a full hookup. When left open, the liquid drains away as you use the toilet and the solids stay behind with too little water to carry them out of the tank, which is how a tank builds the mound that later blocks it. Our black tank guide has the seven-step dump procedure, the water volumes Jayco's manual gives, information about the tank flush, and the tank heaters.

Before you pull in

Put gloves on before you touch anything, and have the hose out and the sequence in your head before you pull up. A dump station is a shared facility, and its etiquette is mostly about the next person: keep the road clear while you wait, and keep everyone who is not helping inside the vehicle. Most visits are quiet and unhurried, so take the time you need; if another RV pulls in behind you, pull forward before you stow the small stuff and let them have the pad.

Connecting the hose

Check the connection at the RV end twice, and put it on like you mean it: grasp the hose by the tabs on either side, twist it into place, and lock the tabs all the way in. Then look at it, because a tab can catch half-latched, especially on the underside where you cannot feel it. The station instructions are blunt about what that costs: "Double check to ensure your sewer hose is TIGHTLY CONNECTED to your RV. If the twist-lock fitting is not put on right, it really can pop off." A clear fitting at the RV end lets you watch the flow and see when the tank is empty. Fittings come in two families, bayonet and threaded, and a universal adapter covers both: Valterra's EZ Coupler fits "3", 3-1/2", and 4"" threads on the park side and twists onto a standard bayonet lug.

Firmly seating the hose in the sewer inlet

The sewer end has to stay put while the tank empties, so seat it firmly into the inlet and then hold it there. The standard trick is weight: "wedge something with weight up against your sewer hose to help prevent it from jumping out of the inlet," as the Central Marin Sanitation Agency's station instructions put it, and a bayonet elbow does the same job with less fuss. On a long run, support the hose so it runs at a slope instead of sagging: Camco's Sidewinder is the common one, and many campgrounds require a support under the hose, so it belongs in the kit whether you think you need it or not.

Dump sites, and what to expect

Use a designated dump station, a campground sewer hookup or a permitted facility. The rinse water at most stations is non-potable, marked for wash-down only and not safe for drinking or for filling a tank, and some stations have no fresh water at all. Some charge a small fee. The rinse hose is for the pad and the sewer hose, never for the fresh tank.

How to treat a black tank

Properly treat the tank after every dump, and understand the two families of RV black tank products and their different roles as tools. Enzyme and bacteria treatments break waste and paper into smaller particles, and they need a clean tank to work: residue from a chemical treatment kills the enzymes, so a tank that has had chemicals in it wants cleaning before an enzyme product will do its job. Chemical treatments do their own work by a different route.

What never goes in is anything that does not dissolve: feminine hygiene products, paper towels and wipes, and household toilet paper that resists breaking down. Undiluted bleach, caustic drain openers and automotive antifreeze damage the seals and the tank, and any formaldehyde-based product is best left out of an RV black tank as well. The black tank guide has the full rundown on treatments and what never goes in.

Storing a black tank for winter

Before the first freeze, empty both waste tanks, black and gray, and pour RV antifreeze down the toilet into the black tank. Only RV antifreeze goes in. Automotive antifreeze is a different product, and it is toxic: it must never reach the fresh water system, the water heater, the water filter or the refrigerator. A coach with a macerator gets antifreeze run through the pump until it discharges at the nozzle.

While the coach is in use, keep water in the toilet bowl: that water is the vapor barrier between the tank and the living space, and a dry bowl is a straight path for tank air and hard on the seal. For winter, run the flush until antifreeze comes through into the bowl, and get enough into the tank, through the line or straight from the jug, to keep any water left in it from freezing. Leave that same antifreeze standing in the bowl the way water normally sits, deep enough that it will not evaporate away before spring: it keeps the bowl valve seal submerged so the rubber stays healthy through the winter, and it seals the tank off from the room the same way water does. In spring, flush it and put water back. The winterizing guide covers the plumbing side, and the black tank guide has the tank-specific steps, including the tank heaters and blowing out a black tank flush line.

Storing the sewer hose

The hose gets cleaned before it goes away, and the gray tank does most of the work. Drain Master's method is to leave the hose connected at both ends while you dump: black first, then gray, and if the gray tank is not full, fill it about two thirds with the taps and dump that through the hose. That lets "the water clean the inside of the hose" on its way out, and nobody has to touch the inside of anything.

At the station, finish with the rinse hose: rinse the sewer hose out, replace the termination cap, and wash down anything you spilled into the inlet. Disconnect at the RV end with the fitting held underneath the dump port, which catches the last of the drips. Then let the hose drain before it goes into storage, with both ends capped. Most hoses ride in the rear bumper or in a storage tube under the coach; if yours has neither, a firm airtight container that holds only sewer gear is the next best thing. The fresh water hose lives in its own bag, somewhere else entirely.

Diagnosing black tank problems

Match the symptom to the page that owns it.

Related guides