If you found no leaks, my guess is the pump is leaking back into the tank. I have seen this happen on all sorts of pumps not just RV ones. Maybe the dealer can replace with a better pump. My opinion is all pumps will do this at some point.
Likely, your low point drains are the culprit as one or both are not sealing closed. Locate them and test whether they click when the close. If not, perform the maintenance in the link above.
Linear Mode
