Should Be Simple But..

My friends Prowler 5th wheel has a unique problem that should be SIMPLE to solve BUT it is not... They have an internal water leak in the area of the furnace, exterior shower.. They haven't been able to pull panels and inspect hose fittings BUT the leak only occurs when connected to park water (55 psi).. if they fill the tank and use the trailers internal pump.. no leak.. without a plumbing diagram hard to tell what goes where but both sources supply about the same PSI??? Just looking for ideas before he starts disassembly..