We have been thinking along the lines of a shower tent and just using our old climbing stoves to heat water, use a foot pump to lift the water to the shower head and viola we can rinse off. Primitive yes, simple yes and easy to repair. Maybe this is the way to go.
The foot pump could be a lot of work.
Solar shower bags range anywhere from 1 to 5 gal. If you don't have the right conditions to use the sun, you might heat the water with the stove as you mention, pour it into the solar shower bag, suspend it, then just adjust the nozzle as needed. Some shower tents can be open at the top, so you set it up under a tree limb, or take some sort of suspension arm to suspend the bag. If you have a roof rack, you might be able to wedge an arm that extends a few feet from the vehicle and suspend the bag from that.
A fair number of Sportsmobile owners use an inverted shower curtain that snaps to a plastic pan. The pan is set on the floor of the living area and the ground ties, now at the top of the tent, are used to suspend the tent from the ceiling. A solar shower bag can be suspended from the roof, or the hose nozzle can be pulled from the sink. Water collects in the pan. One person uses a teak shower pad to stand on in the pan while a small pump pumps the drain water to the sink. Another owner has a flanged drain in the bottom of the pan, which plugs into a hole in the floor (it has port plug for when not in use) from which there is plumbed a pipe to the grey water tank. When the shower is over, the tent folds up into the pan, which is put away, and if it is a floor hole model, the hole is capped.
Some Van owners have a system to open the rear doors 90 degrees and attach a brace from the end of one door to the end of the other, then suspend a homemade cover from above that has a top and 3 sides-two that go over the doors and a middle one that shields the person from view from behind the van. Water just falls to the ground. You could place another brace/bar between the center top of each door and suspend the shower bag from that.
You can find an assortment of solar shower bags online at REI or Amazon, and there are at least 4 different pics of the inverted tent/pan combo on the Sportsmobile Forum. Unfortunately, I have not seen any pics of the system that uses the rear doors.