I remember reading some time ago of a fault in the check valve on the tank side of the fresh water pump. A large batch were bad and were installed on RVs. If the check valve fails it will allow a flow (not a strong flow)through the freshwater system, through the pump and into the fresh water tank. You should have a valve between the fresh water tank and the check valve/pump connection. If you have this valve, close it and see if that solves your overflow problem. If you don't have this valve, then the check valve is going to continue to leak.