You're seeing "poor performance from the OEM shower head because it's the cheapest, low bid item that Keystone could find that "looked good enough to call expensive".... Yeah, I just used "doublespeak"....
As for the "maximum allowable pressure" I don't think I've ever seen an "official recommendation" from Keystone. What I use is the "water pump logic"... My FloJet water pump is rated for 3.5 GPM at 35 PSI. So, I'd suppose that setting my regulator (not my pressure reducer restriction device) at 40PSI or 45PSI should provide sufficient water pressure. If you have a "cheap plastic pressure reducer" they all work by restricting volume to attain low pressure, so once the "trickle of water reaches the shower head, there's not enough volume to "push the water jets to the nether regions.
I'd suggest getting an adjustable water pressure regulator and a Oxygenics Body Spa shower head with remote shutoff valve. They are available in chrome, white and brushed nickel, Here's a link to one source for the brushed nickel shower head.
https://www.amazon.com/Oxygenics-264...gateway&sr=8-3 I'm sure that you can probably find one cheaper, but this is a quick source to provide the information....
As for the OEM shower head, consider it much like the OEM mattress, the OEM stereo, the OEM tank status indicator, the OEM tires and the OEM owner's manual.... All, bare minimum to meet the requirements to say, "Something was provided".....
Oh, and regardless of which setting you decide to use for your input water pressure.... NEVER (ever) leave the campsite with water turned on to the trailer... ALWAYS turn your water off, you never know when a hose clamp or a PEX line might split or come off the fitting..... If the water is turned off, you won't return to a "luxury spa without whirlpool jets" as you open the door.....