I suspect that the tank vent pipe (the pipe that comes down from the roof into the top of the tank) may have slipped down OR is installed to deep into the tank.
The pipe should terminate just inside the top of the tank. If it is installed to deeply into the tank, then once the fluid level covers the end of the pipe it is no longer effective. This is also not good for the tank when dumping as it will cause the tank to crush when draining until the fluid level gets below the vent pipe.
In this situation, what happens when water enters the tank the air has to escape. If the vent pipe is submersed then the air will come out of the admittance valves mentioned above emitting odors into the coach.
Pay attention to exactly when you start to notice the odor. If its before its full, I suspect it is defiantly the tank vent pipe. If so, as long as the top of the tank is accessible its not a difficult fix.
