Finding RV sewage leaks


By Chris Dougherty

Chris Dougherty is a certified RV technician. Here is a letter he received from a reader while he was serving as’s technical editor.

Dear Chris,
toilet leakI suspect we have a leak in our black water system between the toilet and tank, which sits directly below the toilet. I can see the pipe that goes straight down to the tank and it appears that this pipe terminates about an inch or so from the bottom of the tank. I cannot tell how this pipe is attached to the toilet, but that is where I suspect the leak may be. I assume that this is the common set up for black water systems and wanted to see if there is a schematic or other information on just how the parts come together.

Dear Dave,
Each manufacturer assembles their black water system slightly differently. Some use threaded connections at the toilet flange, which also gets screwed to the floor, and the tank, other manufacturers glue the pieces together.

There are several openings in the tank that can leak. If you suspect the leak is at the top of the tank, there are two connections, one for the 3″ pipe coming from the toilet, and a 1.5″ connection for the roof vent.

When the tank comes from the manufacturer it is blank, or has no openings or fittings attached. A hole saw is used to cut the holes, and fittings are then glued in place. The pipe is either screwed in or glued into the fitting. I have seen the tanks and fittings break at this point numerous times. If the tank only leaks when it is completely full, that this is likely where the leak is.


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Roy Ellithorpe
4 years ago

We had that problem with our Mobile Suites. I removed the toilet and found the rubber gasket between toilet and fitting had been bent over when it was installed.