Upgraded rv pumps have 55# discharge
PEX plumbing lines are rated at 100# plus (temperature related)
Reducers are little more than a bandaid....they are fixed, plug up, don't control pressure
Get a good quality 'adjustable' pressure regulator such as a Watts Brand
Set it for 50# (static pressure---no flow)
If hooked up to a system with higher pressure it will regulate to 50#
If hooked up to a system with lower pressure it will fully open and allow whatever pressure is being supplied.
Low pressure at shower.....turn on pump while showering in help increase flow
Remove flow restricter in shower (either in handle or in hose)
Change out hand held shower to an Oxygenics Brand