pstandiford is right. When you take off the cap on the city water connection, look inside and you should see a washer with a debris screen imbedded in it. That screen serves two purposes. It blocks debris. But it also depresses a check valve in the city water connection on the RV when the connection is tightened on the hose. The screen pushes the check valve open to allow city water to flow into the RV. When the city water hose is disconnected, the check valve closes so that water doesn't flow out the city water connection when you run the RV water pump using the fresh water tank.
Your check valve in the city water connection is probably stuck open.
Ed & Bev Felker
Retired USAF Col and retired Nurse
Traveling with Lily & Bella ('Teddy Bear' breed)
2015 Vegas 24.1 (E-350)