Dick and Trish,
When we are hooked up to city water, the pressure is real good. When we are running independant, the water pressure is significantly less, but still generous enough to meet our needs including the shower.
Is your problem when on city hookup, when independant, or both?
If both, maybe the shower faucet or Hose-With-Shower-Head has sucked in something creating a partial blockage.
If only when being independant, I heard that some motor homes (all brands, not just Phoenix Crusiers) the fresh water tank is not cleaned out adequately from the scrap left behind from drilling holes into the tank for the filler tube, drain tube, outlet pipe, and gauge sensor. The loose tank plastic can get sucked through the tank's outlet port into the interior plumbing, clogging or restricting something. Back pressure can usually force the debris out. You would have to disconnect the plumbing at the pump to dislodge it from the system.
There may also be a restriction in the tank itself blocking the outlet port at that point in the tank. It can be a challenge to remove any such bebris.