Two points that I would expect to find.
One is that you may be missing what you are seeing on the PEX to faucet connection as I would expect to find a fitting somewhat like this:
https://www.homedepot.com/p/Zurn-1-2...E&gclsrc=aw.ds
This is in colors making it easier to describe what I would expect.
Somewhat like a hose connection in that the center black portion if clipped pretty solidly to the PEX but the white outer portion should turn. Maybe with some struggle as it may have set and semi-frozen in place?
So if there is a leak at the connection from PEX to the male threads on the faucet, I would expect to possibly find a rubber cone washer maybe and then the "nut" could be tightened up or at worst the rubber part might need to be replaced if it is old and crumbles with age.
I have not looked at the single handle shower faucets for whether it comes out the front, so can't help there.
First I would check for just a bit of tighten to fix the leak? Use care with the plastic so that you don't over tighten to break it!!
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Richard
Why no RV year, make and floorplan on MY signature as we suggest for others?
I currently DO NOT have one!