I've never done an RV bathroom but I've done ones in my homes, along with other home renovations so this would be my perspective- scorched earth. Do a complete tearout, then rebuild it into exactly what you want. Sounds horrific and unnecessary but from experience you're not going to surgically remove and install just a new shower without a lot of tedious work and frustration. The fixtures in an RV are low grade and hard to justify going through any measure to preserve them. Take lots of measurements, come up with the new layout and materials, line up any custom cabinetry or carpentry you may want and just go for it. It will turn out way better, and it doesn't have to be very expensive if you don't want it to be. The work will be straightforward, standard demo and installation techniques which will go much more quickly and easily. You can deal with the analysis paralysis of the shower surgery if you want but there is NO WAY I would approach this any other way than a complete renovation.
Mark B.
Albuquerque, NM