It is the hole just above the fill hole. That is not to prevent over filling. It is so that air can get in and out of the tank. If you have seen water come out of this when you fill, it can have water sitting in a low spot that will prevent air from getting into the tank. Try removing the fill cap and running the pump to test this. Of course if the fill hose has a low spot this may not work ether.
Dave & Barbara
2005 Mobile Suites 38RL3 #2338
MorRyde IS 8K axles, LazyBoy recliners, LCD tv
Full time since '96