One person said he's never seen a campground that allows washing....he must only camp at one campground. For 40 years we've camped all over the west, with a couple of trips back east. Some campgrounds allow washing, some don't and some will after paying a fee. You'll find very few in California because of the water shortage, but states with lots of water don't have an issue. As a matter of fact, I can't think of any campgrounds in really wet states that prohibit it.
We keep ours at home and it's washed before every trip. If we're out for more than a week and the coach gets dirty, I usually wash it. When it's allowed, I try to do it without being obvious and ruining it for others.
No matter what the rules say, I always use a hose and brush to wash off the front of the coach. They don't usually have an issue when it comes to the safety of having a clean windshield.