I wash mine when I can but usually once a month. The roof gets washed too..you wash your hair when you shower don't you...:S..the coach, including the roof, gets waxed twice a year. Takes me a couple hours at a campsite to do the wash. Some CGs charge $10 others don't care. If it's going to rain, simply lather it up and let Mother Nature rinse it for you. It still needs to be wiped down with microfiber towels. Make sure you wet the microfiber towels before drying...washing your coach gets you up close and personal with your rig. You will be surprised at other things you find while washing it. Things that you can easily repair/fix that you might otherwise miss.
Unless you use soft water, you will get water spots if you don't wipe it down after hosing it off......Dennis