Ecept for the fact that you need to pump water up to get a shower, I've found is the pop-up types work well for privacy rooms. Shower, toilets, changing room. There are dozens of brands on the market.
These stow into a couple of feet circumference circle bag and are only a couple of inches thick. The only trick is folding them; once you've done it twice or thrice, it becomes relatively intuitive. There are videos on YouTube which demonstrate it. It's not going to break --- trust the video
If you've ever folded a bandsaw blade, the principle is the same except there are no sharp teeth to lacerate your spleen
The downside is that they do not have the structural strength to hold a bag of water. Using an on-demand hot water heater is quite nice, but you'll burn through 8 gallons of water quickly unless you have a pump and a water cube, which starts to defeat the purpose of a lightweight shower room.
If you don't need a lot of water (short hair) I've seen garden pump sprayers set up as a water source. I think that is a clever idea myself. If you do have longer hair, spray 'dry shampoo' works very well --- goes on like a heavy hair spray. I am told baking soda works as well, but I have not tried it persaonlly.
If you have a stout constitution, and warm days, the water from the onboard water supply is usually relatively warmish, especially on an 85- or 90-degree day. It's not bad. You just need some tubing or a lightweight hose with a showerhead that has an off/on valve.
I use this more for the porta-potty than I do a shower, but it will serve well for both.
I've gotten used to GI showers, I use very little water and can execute the shower discreetly either inside the cabin. "Dude Wipes" are pretty good as well for average days.
