It depends on the manufacturer of your particular inlet valve.
Most involve total replacement. Remove screws slide a knife behind the plastic face and cut away sealant. pull unit out and remove hose; install new unit to hose; re-goop and screw it back into place.
A temp fix can be made with a hose check valve screwed into the city water port.
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW