Refreshing cllosed fresh water system

I have a 2015 3611rl Montana with a closed fresh water system, How do you sanitize the fresh water tank. There is a vent located at the rear of the camper to vent the system, this is where you used to fill the fresh water tank, but now you turn a knob at the city water connection to transfer water to the tank. Getting bleach back to the tank is a problem.