This has been covered on here many times, but here's the short answer:
When connected to city water, the cold water lines and the water pump lines join so it's a transferless system when going from city to onboard water and back. Therefore there has to be a mechanism to inhibit the city water from going past the pump and out the suction side into the fresh water tank. Normally there are (is) check valves in the body of the pump to prevent this. A lot of us have had this issue of these check valves leaking..........the result is then the fresh water tank fills from the city water connection. In most, if not all of the Redwood models, there is a quarter turn valve between the fresh water tank and the inlet of the pump. It's normally used to pipe the inlet of the pump to the winterizing hose--------to stick into the RV anti-freeze. I just close that quarter turn valve all the time, as we usually are only hooked up to city water. If you open that small door in the rear basement wall (on ours the street side) you should be able to see it with a flashlight. I can operate ours through this small door, but if you can't you can take down that one panel to access. If you still want to use on board water you can either turn the valve back on or replace the pump with a new one.
Jim & Mary 2013 36 RL with Titan Disc Brakes Residential Fridge Goodyear 17.5 H rated tires MORryde IS, MORryde pin box 2015 Ford F350 King Ranch DRW