I have an indoor shower on my Phoenix pop-up. I had it custom built into the floor. It's a shallow pan that drains to the outside. Depending where we are, I'll either connect it to a hose that leads to a tree or to an external tank. Originally, it came with a cover for the pan, but it became a hassle to remove the cover and shower. I've since made a wood shower mat that allows the water to drain. The curtain is hooked around the frame of the "escape hatch" for ventilation (no condensation issues).
A few issues with my set-up:
curtain is custom made and bulky. Takes more space than we'd like.
Dirt and debris get's into the pan - more-so now with the shower mat. Just need to clean up before showers or it will clog.
Curtain and shower head are too close to our wave3 heater. So if we're in a colder area, we need to turn off and cover it before showering.
Other than that, we're pretty happy with it and glad we didn't go with a wet-bath. We were used to doing solar showers in a larger rubbermade bin anyway.