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
2006 Winnebago Outlook 29B E-450.
2012 Honda CR-V AWD
Blue Ox Aventa LX tow bar and Brake Buddy Vantage.