Beverley&Ken wrote:
There is a one way valve just inside the city water hookup, actually it is part of the hose input. Inside the fitting is a one way valve that sometimes becomes dislodged, Most common after winterzing.
The part us not expensive, approx $10. Easy to replace, on the outside of the RV, remove the screws holding the inlet, pull out slightly and replace with the new. New caulking and done.
Ken
This is true; and they make an all plastic version (which your probably have) and a superior version with a brass connection at the backside.