With the limited use the shower in a little camper gets I think a premium latex enamel - gloss or semi-gloss, would work just fine. I'm not sure what they were painted with in the first place but a few coats of Kilz and and enamel would be really sturdy.
I glued foam insulation to the exterior walls and painted with Kilz and semi-gloss. 10-15 showers and 5,000 miles and no problems so far.
A two part epoxy applied with a brush or foam roller would work well. Open all the doors and windows
and set up a fan to blow out the door. That paint is very durable but very expensive, I'd rather repaint with latex every so often...
I pulled the shower pan and sprayed with appliance epoxy - not an inside job. After cure no odor.