I look at quite a few YouTube videographers. A couple of them live and travel in vans. Their method of showering is to get a new 2 gal sprayer bottle (Yard maintenance type), remove the wand/nozzle, splice a kitchen sink spray hose and nozzle to the bottle hose. Some paint the 2gal jug flat black leaving a 1" unpainted vertical line in 1 area so that the sun will heat the bottled water & the unpainted stripe allows one to see how much water is in the jug.

Besides being an emergency shower, this would be very useful on any kind of outing for washing hands, dishes, pots, pans, etc.

Just put van camping shower in YouTube's search engine. You'll have to sort through the videos that come up but there's at least 2 that cover the bug sprayer shower system.