IN WWII, soldiers in Africa would put their clothes in a bucket with some water and soap, attach it to the bumper of the truck, then drove all day on the rutted out roads.
Plenty of agitation. At the end of the day, just rinse and hang out to dry. Maybe the same thing would work with a 5 gal bucket on the rear bumper or near the
propane bottle of a camper? It wouldn't be nearly as bumpy as a rutted African road, but it might be enough. And, nearly free.