if the water system was not messed with, you have either external or internal water supply. Once you hook up to an external water supply (yes an additional pressure regulator is highly recommended), your fresh water tank will not be filled, since there are 1-way check valves in place. The same holds true if you use your waterpump, a check valve will keep the water from running out your city water hook up.
to flush the fresh water tank, you need to find the drain valves, close to the water tank. Open them up and let the tank drain completely. Then partially fill the tank and add half a cup of chlorox. if you have the rear bath, your tank should be under the drivers side twin bed. Its a good idea to check if the tank is still mounted properly while you are looking around there.
1983 310 Classic Motorhome
1997 30' Excella Trailer