Fresh water on 1979 Airstream Excella

[Walkietalkie]

This motorhome is new to me so I am trying to figure everything out and I don't want to wreck anything. I am at a rv park and I have access to city water. I also bought a pressure regulator and hose so I am ready to hook up. The part I am not sure of is exactly how to do so. If I just hook up the home to my RV and turn on the water won't that fill the fresh water tank? Is there some way to flush the tank because I don't know how long the water that is in there now has been sitting. Is there a switch or some valve that I need to change when I'm hooked up to city water? I am looking for somewhat of a step-by-step here. I've added a photo of what I believe to be the fresh water hookup.

[PeterH-350LE]

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.