If you do it discreetly, you can probably do it just about anywhere. Mostly RV parks and campgrounds will have a "no washing camper" rule so you won't run a garden hose and use excessive water, and flood the campsite, or your neighbors campsite.
But absolutely every campsite I've ever been on, I've taken a pan of water, a wash cloth and washed the side of my camper, usually the slide out slides. I spot clean, clean the awning all the time, and never has anyone ever challenged me on it.
I've been on the roof of my camper checking things out, and I've seen other campers doing repairs on their roofs. But it's always done quietly, without any fanfare, and without making any kinds of messes. For example, if you are fixing a spot on your roof, you don't drag out of the back of your truck a construction company! It's done discretely and very quiet. No one will ever notice. And absolutely no evidence laying around the camper you are doing a repair or a fix. It's always tool in hand, or out of site, never laying around. This works, and I've done my share of instantaneous fixes at campgrounds everywhere. Never been challenged.