New to RVing... need help with fresh water filling

Many of the MHs have that valve system. Mine has it also. I am sure from brand to brand or year to year there may be some differences but when I am on city water I can get water at the fauset. I don't open any valves inside. There is an overflow for the tank. I just leave all valves shut off and turn the valve and it starts filling the tank instead of furnishing it to the fausets. I am wondering if there is a check valve that isn't working because you are opening valves inside the coach.