I will describe several Pressure Reuglators most of which I have tested (one exception)
Standard RV water pressure regulator.. Brass or plastic cylinder about 3/4" diamater, male hose fitting on one end, female on other.
This unit will limit pressure when not drawing water but when you try to draw water.... It seriously restricts flow..You will not enjoy a shower if your RV is so equipped.
HIGH FLOW or HIgh Capacity.. like above but with a bright plastic grip on the female end or wings on the body... Better but see above shower
SUR-FLO.. IF your RV has one Built in, this is it... This one can handle a standard RV Shower's needs when new.. Like the rest of this list as it ages the max flow rate decreases, and as all but the one I have NOT tested (last one) They can be rebuilt.
Watts or Zurin "Whole House" sold in plumbing supply and a few RV stores, Min wer 3/4 inch models (Cheaper than 1/2 inch models) with added garden hose adapters (sold seperatly) they are adjustable and can easily feed TWO RV showers at once (you will need a Y connector to do that
). You may wish to buy a pessure gauge..I suggest it NOT be a Watts. (Great regulators, lousy gauges).
And the one I have not tested:
Valterra Adjustable.. Others have and say it is as good as the Watts. above