It seems to be in slightly diffrent places on diffrent years but, it should be in the first 2-3 feet of pressure line coming in from the city water connection. It looks like 2 brass bells mounted bottom to bottom, with a hex bolt sticking out of the top. It is the pressure regulator and the check valve to keep your water system pressurized when on the pump. There should also be a presser pop off valve near it with a hose that runs thru the floor. The pressure vlave screws into a T fitting and is round, about the size of a quarter. It is brass as well.
As to using a pressure regulator at the campground faucet, it is a good idea. Even with the regulator in the unit, you could burst a water hose and have it beat your trailer up if the pressure was high enough.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato