I have the same design problem.
Changing that shower faucet looks to be the hardest job (not involving delamination or roof replacement) on the camper.
Crawling up through the basement in my 5th wheel to access the innards of the sink base to gain access to the back of my shower seems excessive. If I ever need to do it I think I will be removing the kitchen countertop and sink.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW