Here's my solution to a space-saving bathroom/shower. It's doesn't require waterproof seals, moving walls, etc.
Start with the recognition that a shower takes a lot more space than using the bathroom, but you just need a curtain, rather than the solid walls you would want when using the toilet (if someone else is in the van with you).
Build a 2 foot wide, 2.5 foot deep closet-shaped toilet room, with split doors opening out into the hallway. Install a port-a-potty or composting toilet in the back of this . This is plenty of room to use the toilet. Install a fan in the roof of the closet.
Granted, 2' x 2.5' is definitely precious real estate inside a van, but you can also use the space to store a trash can, hang stuff, put bulky items, etc. when not in use.
To take a shower, open the doors, place a 2' x 2; portable basin on the floor (with a draining mechanism, or not, depending on how fancy you want to get) , and open a shower curtain on a looped roof rail that goes from the back of the toilet closet (curtain draped over the toilet to keep it dry) projected out 2 feet into the hallway. This gives you a 2-foot wide, 4.5 deep at the top, shower enclosure within the curtain with no moving parts or seals needed. Plenty of space to take a shower.
Once you are done, allow the curtain to dry and then tuck it away or take it down, and close the split doors.
Lots of room both both functions and total privacy when using the toilet.
I think that creating moving walls and engineering waterproof seals seems way too complicated.