How much should I charge an artist to paint a mural on my motor home?

It is not clear why you expect to be paid for someone to paint a mural on your RV. If the mural is an advertising, the advertiser likely has a set, flat rate for people who agree to allow their RVs to be painted. If you are commissioning the artist, the artist will determine the cost of the project.

Well, you wouldn't want to charge an artist anything to do work for you. You can expect an artist to charge you up to several thousand dollars depending on the size of the mural that you want painted.