If we visit the several usual campgrounds, they let RVers wash their rigs.
Some State and County Campgrounds will let you wash them but there are probably plenty that don't.
We've found several caamp grounds won't let you wash your rig but a RV detailing or washing outfit is allowed because they get a big kickback.
Usually we just wash the windshield and front end if there for a day or two.
I hate t wash it or do now because it is so hard on us.
If it is going to rain and it is really dirty I have sudsed it up in the rain.
I hate a dirty rig as much as I hate a dirty car.
Used Blue Beacon in Ky once. Not clean in areas and took the wax off.
Ended up washing it the next day when we got to where it was warm enough.