If this happens on city water and pump, I would not think it was the pressure regulator, as that is typically only in line with the city water connection. If I read the symptoms correctly, it sounds like a restriction - pressure builds up downstream of the restriction, and when water is used, and the supply can not keep up because of the restriction. Turn off the shower and the pressure builds up again. Rinse and repeat.
Should be easy enough to tell if it is just a shower issue, or if it affects the other outlets in the coach - bathroom sink, toilet, kitchen sink, shower, outside shower. If it affects all of the outlets, it is something close to the source. If it is only the shower, it is in that specific circuit.
One of the first things I would check is the outside shower. I have seen very odd things happen with water pressure if the outside shower faucet is turned on, but the sprayer head is turned off. Even a little bit of "on" can cause oddness. Take a look there and make sure that both knobs are all the way off.
2006 Monaco Knight 40PLQ | 2011 Jeep Wrangler Rubicon Unlimited