It's the WA State Patrol faq. http://www.wsp.wa.gov/about/d5faqs.htm

Yes, as long as all seat belts are in use-although no one under sixteen may ride in the back.

I think that means you need to install seat belts. In most rvs, they are at the dinette seats. I don't know about your friends particular injury, but walking every day helps everyone that I know and my self although it hurts to start.
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Wear a PFD if you want to live.