Do you have a separate outside manual water fill?
If not, for RVs without the outside manual water fill they use a manual valve inside the RV to fill the water tank. That valve may be open slightly allowing water to slowly fill the tank.
If you do have outside manual water fill, the only way water can get into the fresh water tank is if the 12V water pump check valve fails..
In this case you need to check the hose on the input side of the pump (which goes to the freshwater tank) and see if there is water coming out of the input side of the pump with the city water pressurized.
If water pump check valve has failed, you might be able to get a rebuild kit or if you can't find parts you will need to buy a new water pump..
If you don't use freshwater tank at all and have no intentions to ever use it, you could add a manual shutoff valve on the output side of pump and simply turn off the valve.