If you are going to bathe in a river or lake make sure you bring along the environmentally friendly body wash. It is non-toxic and rapidly biodegrades. You can get it at any camping store. Back in the day they used to recommend plain original Ivory bar soap as being the most biofriendly of the "regular" soaps due to the lack of oils, colorants and perfumes. Just going for a swim and washing using a handcloth without soap will get off the sweat and most of the dirt and will suffice for a day or two.
http://www.vermontsoap.com/press/press7.html
Beware of using a lot of deodorants, powders, etc to control odor as they can also become very irritating to skin when you are sweating alot and have limited chances to shower.
In addition to truck stops, many health clubs, swimming pools, campgrounds and other facilities will allow you to use their showers for a minimal fee. Check ahead with cycle shops or clubs along your route as they usually know of cycling friendly stopping spots with amenities.