The pump acts as the check valve to the fresh water tank when hooked to city water. There should be a small door, on the curb side of the trailer, with a fill point behind it to fill the fresh water tank. Some years did have an "auto fill switch" on the control panel that would allow you to fill the fresh water tank from within the trailer while hooked up to city water, around 1991 or so. A very nice feature.
The balance of the water system is straight forward plumbing.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles