The two 3-gallon shower bags we got for Christmas are working perfectly. It's sunny enough up here we could probably shower every hour from 12-4. Morning showers are cold, but maybe we'll experiment with putting hot bags in a cooler or sleeping bag and see how long that keeps the heat.
The only strong and tall enough place to attach the shower bags so far is the van. Yesterday I installed a tarp as a privacy wall, but the wind blows it up so these showers are only private if neighbors aren't trying to see what we're up to. So 3 gallons is more than enough water for a shower because we're rushing. I think I'll wash my hair after dark.
Soon we'll erect a teepee-style shower cabana which will solve the privacy issue but also provide flexibility, since we can carry it wherever we want to water the land.