I strongly recommend Blue Beacon Truck wash for RVs. They are all over the place and charge $37.50 for motorhomes over 30' like mine and only $24 for trailers 20-30' long. Here are prices for other RVs:
https://bluebeacon.com/our-services/rv-motorcoach/ They might be a little hard to find sometimes because they tend to be behind one of the larger truck gas stations, like Flying J, but just follow the signs and you will find them.
You may have to wait in line up to an hour, but I usually plan on fixing and eating lunch while in line. You line up with the big trucks and just keep moving up until it is your turn. When they guide you in, you get out and go into the office to pay. You can leave pets and other people inside. As you pull in, a bunch of guys with wands and brushes swarm over your vehicle and get it pretty clean. They rinse it, but do not dry it. You might have to do your windows with some cleaner and paper towel when you get to your location, but most of the time I don't bother. They do a very good job of getting bugs and grease spots off, which is very good.
All in all, it is a LOT cheaper than paying someone to do it in an RV campground.
I took my grandkids with me once, by the way, and they loved it!!