My immediate solution is to turn on the water pump and let it add pressure to the line when taking a shower.
My second suggestion is to change the shower head to the
Oxygenics shower head sold at camping world, etc. I have this one and am very pleased. I do know one person that does not like it.
Third there may be a restriction device in the faucet area or the end of the shower hose. I can not verify this is the case.
I hope this helps and safe travels.