Along with the pressure regulator (don't get one of the cheap plastic things), you might consider some kind of filtration. It can be as simple as a single in-line filter, or a two-bowl arrangment like mine. When buying a regulator, a key consideration is what the flow rate, or gpm, is. The little plastic ones will greatly restrict the flow into your rig.
Even the brass ones can be junk. Mine only lets 40psi thru. Getting a new adjustable one this winter.
Where can you purchase an adjustable regulator? I went to camping world yesterday and they didn't have any.
We bought a 2009 Repo,ed Bighorn 3670RL and have no manuals other than the generic one off the web. When you hook up to city water with a hose, can the water be left on, or does it just fill the tank?