You by chance didn't hookup hose to black tank flush vs city water?
I think the valve you are referring to is the backflow preventer on the black waste tank flush line and you are filling your black tank.
Do you have a gravity fill for fresh water tank OR a water panel with switches/valves?
Gravity fill....then you are hooked to black tank flush.
If water panel........check position for switches/valves. Set to fill tank will cause water to flow and nothing inside rig to faucets etc.
And don't turn on your water heater until you get WH tank full and water flowing out of faucets.