I think Geronimo73 has it right. Where you screw in the white hose going to city water on the RV -- look inside where you screw in the hose on the side of the RV. You should see a rubber washer with a screen.
When you tighten the hose connection, that screen will push in on a check valve allowing water from the hose to flow into the RV's plumbing. That washer/screen is there so that when you use the fresh water tank and water pump, you don't go blowing the water out the hose connection. With no hose connected the screen will not be opening the check valve. The closed check valve ensures water does not flow out the connection when the water pump pressurizes the system.
If the washer/screen is missing, the system will work fine using the pump and fresh water tank. Connecting city water without the washer/screen in the RV connection will not work since water can't get into the plumbing because the check valve is closed.
https://www.amazon.com/Superior-Elec...%2C199&sr=8-47
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