A simple shower enclosure I've thought of would work like so and be made of 3/4" thin wall electrical conduit tubing.
Over the left rear wheel, two 1" eyelet bolts would mount into the wall of the Van. (these could be stainless steel or cadmium plated) set about 3 ft apart. Just below them large fender washers could be attached to the wall of the van to reinforce the thin sheet metal. The black square in front of the left rear wheel would be the shower curtain rod. The ends could be set down through the eyelet bolts and the ends brace against the
reinforcing washers. A simple light weight shower curtain could be hung from this to form a privacy enclosure. The shower curtain may be installed onto the square enclosure rod first even before mounting into the eyelet bolts.
There would be no drainage, so a piece of plastic would need to placed on the ground and perhaps a pair of flip flop sandals be used for walking out of the enclosure to keep one's feet clean after the shower.
This assumes you are in warm enough environments to utilize this simple system. If one is boondocking it could be left in place until ready to travel. Then taken down and stowed away easily enough in an out of the way area in the Van. If one carries a chemical toilet and the shower curtain is long enough it could provide privacy for that too.
As for the shower water a garden sprayer painted black could provide enough water for that. It has been discussed here before to mount a kitchen sink sprinkler to the hose after removing the spray wand from it.
The plastic base to stand on may be the lid of a storage tote box.
As for the flip flops....a strip of plastic magnet glued to the base wold allow them to stick to the side of the Van
just outside the showering area. A small container suspended from the shower curtain rod by a piece of cord could hold soap & shampoo. Similarly a locker mirror could have have plastic magnetic strip glued to the back of it and mount to the side of the Van for shaving.
But this is a system I've thought out.