[rverdlm]

Since the only thing that isolates the fresh water tank from the city water is the pump, I would suspect that the pump membrain is ruptured. Try taking the pump input line of the pump and see if water comes out of the pump. It should not. The only way water can get into the tank is through the fill hole which is not pressurized or through the tank output line which should go directly to the pump input. So unless someone reconfigured the the coaches plumbing this should be the problem.