Truck stops, campgrounds and even Hotels may allow you to use showers for a fee.
Fresh water is available wherever there is a spigot...you just need to get permission to fill your tank. We have stopped at gas stations, fire stations, city parks, truck stops, even farms and asked to fill our freshwater tanks. We offer to pay but most times all have said "go ahead and fill,keep your money"....
I do carry two collapsible 3 gallon water jugs in case we need spare water. We also have a solar shower. It's a 1 gallon bag with a hose and shower nozzle to use for boon dock showers. Let the bag full of water heat up in the sun, put up a tarp for privacy and take a shower.
When travelling on a several week trip we typically boon dock several nights and then stay in a campground one night for water, showers and to dump. Then back to boon docking for several days and then stay in a campground one night...