Your fresh water tank has nothing to do with the city water system. The water in your fresh tank had to be put there separately from the city water connection.
Only if everything is working correctly. If the check valve in your water pump is leaking, your city water WILL fill the FW tank.
My Roo had separate valves on the suction side of the pump, a normally open one to the FW tank line and a normally closed one to the winterizing connection line. If you are set up like this, you can shut the valve to the FW tank and it should stop any filling from the city water.
Instead of 2 valves, some units have a single 3 way valve that can line the pump up to either the FW tank or the winterizing connection. I ASSUME it lines to either one, so if you swapped it from the FW tank to the winterizing connection, it wouldn't fill your FW tank, but may dribble out of the winterizing connection. I don't have one of these (that I know of - haven't checked out my Mini Lite arrangement yet), so I don't know if it's possible to align the valve between the two suction sources and thus shut them both off. Someone with experience with one of these 3-way valves may comment.