rdhetrick wrote:
I'd guess the water pump was left on and the toilet fill valve was leaking by.
That would only be true if he left the fresh water tank full while the TT was in storage, and there was more fresh water than the black tank could hold.
You might be missing one or both caps on the roof vents. If you look on the roof there are two 1-1/2" PVC pipes that come up thru the roof and have vent caps on them. One is black tank, one is gray tank. If the cap was missing, rainwater and/or melting snow will run down the vent and fill up the tank. Once the black tank is full it will overflow from the toilet. The gray tank usually backs up into the tub/shower since that's the lowest drain.