A 2.5 gallon solar shower heats well in the sun and does a great job outside for shampoos....or suspend from a tree in a secluded place and have at it.
I bought some of these at WalMart last year, prior to a boondocking trip in New Mexico, and they provide an excellent bath, needing just a small amount of fresh water for a quick rinse.
Comfort Bath Heavyweight Cleansing Cloths
I couldn't get a picture to copy, but they come in packs of 8, are very wet
, and then eliminate the need for water to rinse a washcloth, as you just throw them away.
You get them in the area where they sell Depends, and other such items.