72 Tradewind Pump works but goes out the back
It sounds to me like you have a pressure regulator valve stuck in the open position. If your coach has the one-stop service compartment in the rear, you can usually see the pressure regulator valve when the one-stop hatch is open -- it is brass and has a top that is somewhat bell shaped. These valve should be readily available from your local plumbing supply house.
Once the problem allowing water to flow out the city inlet is solved, your pump should only run when a faucet is open, the toilet is flushed, or if there is a leak somewhere in the system.
Good luck with your investigation!
Kevin D. Allen
WBCCI (Lifetime Member)/VAC/Free Wheelers #6359
1964 Overlander International/1999 GMC K2500 Suburban (7400 VORTEC/4.11 Differentials)
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado Convertible (8.2 Liter V8/2.70 Final Drive)