Truck stops are good places to take showers, they cost about $10 give or take a dollar, and sometimes you have to wait your turn.

If you have to take a shower or bath outside then the best method is to collect water in a portable plastic container and wash at least 200 feet from a river, stream, lake, pond etc, and use Dr Bronner's 18 in 1 hemp pure Castile soap, this stuff is supposedly the least likely to have any effect on water and you can get it at Walgreens, Target, and Meijer. Then if you have any gray water after washing pour it on the ground at least 400 feet from any water source. Or unscented baby wipes or towelettes or sanitizing wet wipes which kill bacteria which causes odor, or washcloth and baking soda and hot water.