I like all of Mary and Don's ideas, although I'm fine w/o a sink in the shower.
In the past I've brought up the idea of having a wet bath like a popup's, where the hard walls only come up to your knees. I was thinking of doing it as a podmod, but a factory option would be much better. The advantage of the low walls is a soft curtain would surround you and it could even taper outward as you go up. This would give you MUCH more elbow room and room to bend down. You bang your elbow, you hit a soft curtain and not a hard wall. You can even push it outward to temporarily make more room as you soap up. Secure it at the bottom with commercial-grade velcro in a half-dozen spots, and up the seam to seal it shut (same stuff they use with the EZ-pass to stick it to the windshield). When you are finished roll the curtain up from the bottom and secure it to the ceiling, which will then make the 'pod feel much more "open" since the walls aren't in the way. Toilet use? If everyone is outside just lock the 'pod door. If others are inside, roll the curtain down. Its just as much "privacy" as the current walls and door - not that you really have any privacy in such close quarters. I really miss this setup from our popup.
Ummm, no thanks on that storage idea. I wouldn't have a problem if this was only a shower and I had to take it down once a day for 20 minutes. But there's no way you could use the toilet w/o removing it and that's not going to happen. Especially with 4 people. Much better idea to add shelves/cabinets to other areas of the camper where there's room. In our 171 there's still room to the right of the kitchen above the window, and above both windows in the rear bed area. Possibly above the other window up front but that might make it feel too small.