Damn it. I have an 89 Dodge 360 camper van with 38,000 miles I'm putting heads on in the shop right now, what a gigantic pile of chit, even Hagan asked "Who would own this?" laughing1
You can probably find a cheap camping trailer from 80's to tear apart and use, really all you need is a shower pan, water heater and a holding tank from one. I'd use cpvc to plumb it, and some frp panels glued to the walls. Get fancy and put a ceiling vent in to keep steam down.
The main thing to think about is will you be using it hooked up at a park RV spot or stand alone, self sufficient like. The latter will require a lot more parts to make it work.