We are on the road all the time, we have no residence. Most RV parks will not let you wash your RV. I have used truck stops that charge from 75 to 125 dollars and do a terrible job. What do most do?
If it's been a particularly muddy or dusty trip I'll use the pressure washer at home to knock the big chunks off. Just be sure you don't use too tight of a nozzle. Beyond that driving thru rain helps keep things clean enough.