A solar shower bf will be the solution to your problem. They aree available in sizes from 5 gal (rather heavy) to 2.5 gal(just right) to one gallon (barely enough), available in sporting goods stores or mail order (REI definitively).

Fill the bag, put it in the sun in the early AM, an enjoy a nice warm shower about 6 PM or earlier. Alternatively, heat water on the stove and fill the bag.

I have used these for weeks at a time on field projects with complete satisfaction.