Now that I have a 5er it may be different now, but when I had my previous TT's, I'd simply get a sponge or a soft cloth, or even paper towel, and simply wash a very small area at a time... like maybe or 3 feet, or from one window to the next, or one slide out, at a time. I'd do a little more the next day until I got completely around the camper. It might have taken a week to do it that way, but eventually the entire camper got washed.
I did this last year over a 2 week trip. Every day I did a little. No hoses, and no obvious fan-fair. Start by cleaning the outside of a window, even if you are on a ladder, then simply move a little farther. The secret is not to draw attention, and not to drag out hoses. Wipe the soap water on. Wipe the soap water off, move a few more feet and repeat. Let no water drop on the ground, and you can do it this way.
Now, my new 5er is so tall, I'll need a fire truck with the extension ladder to reach the top. So, I think my days of doing it this way are over.