If you are filling the tank it is not "city water". That is done by connecting the city hose to the inlet for city only water connection. If you are using the tank fill it is going into the tank and will only run off of the water pump. If the pump is off the water will not come from the tank and since the line would not be hooked to the "city" line no water is coming in that way either.
Simply put, either you have a hose connected to the "city" connection which is likely a simple hose connection on the outside of the trailer, or you have filled the tank from the tank input, which is a different connection and are running from the tank with the pump.