Had this very same problem this past season. We would drain the tank, flush it with the built-in flusher, and the next day it would be full to the top again. When we looked down into the tank, all we could see was white and water rising up near floor level. It looked like it was white PVC down there with a bend in it. Tried sticks and a piece of hose to clear it out. Nothing worked including pumping it out. Even got the portable tank out once just before midnight and partially drained the black tank only to find that the tank was plugged again in the morning.
In desperation, we used liquid Drano which does not harm plastic or metal (ie, the sensors). That did the trick and it looked clean enough down there to eat off (if you could or wanted too
). There may be some other chemicals around that will work as or nearly as good.
It turned out that whoever glued up the pipe at the factory used waaay too much ABS glue and it was hanging down around the pipe like stalactites and there was puddles of glue on the tank bottom. This was trapping the TP in there and there was nothing that could move it out of the way. For the first time, we could see the tank flush head next to the pipe into the tank and for the first time, our black tank sensors read zero. Very exciting! Pulling the toilet and trimming off the excess glue was the fix for this. It's the last thing I would have thought of and took an entire camping season to figure out. It wasn't until we were at a cg for a week with no sewer hookups that the problem came to a head. We had the honey wagon pump the tank out 2 days in a row and we had to use the nearby toilet building most of the time. Not cool... Spend tens of thousands on an RV and you have to use the public restroom?
If you do decide to try the drano method, you MUST use the type that is septic safe and won't attack plastic or metal. Then NEVER drain it into an on-site septic system because it will destroy the disposal field by killing off the bacteria in it. This can cause an owner many thousands to fix so make sure you don't do it. Many cgs have on-site disposal systems so beware... Do this at home if on a municipal system or at a dump station somewhere on one. Put the Drano in before you leave the cg and let it slosh around on the way home.
If even this doesn't work, you *could* buy one of those cheap USB cameras that can be pushed down a pipe, to find out what it's like inside the tank. Can't recall what they're called exactly.
Another point is that although there are a number or "RV" TP brands out there, some of them do not disintegrate like they should. I did a test of several brands last year and one popular RV brand (Thetford IIRC) didn't disintegrate. Some ordinary households brands DO disintegrate like an RV type should. One Costco type does and there are others.
If your RV type TP doesn't disintegrate, it will aggravate your plugging-up problem. There does not seem to be any standard out there and anyone can call it "RV" TP. If the glue is the problem, if you pull the toilet out, you should replace the gasket even if it looks okay.
FWIW, we never could see the bottom of the black tank from day one. Never thought twice about that. In retrospect, I'm positive that the problem started with the delivery guy during transit from Indiana to the west coast. I'm guessing that they likely don't spend extra on the RV stuff. Then the dealer was lazy in not checking and cleaning the tank?
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