It's illegal in California. I'm not sure if it's allowed anywhere, though. But even if it were, I think it wouldn't really be a good idea to allow someone to ride in the RV while it's being towed on the road, even if it is just for a quick nap. Accidents can happen, not just on the road but even inside the RV while it's being moved. Best to just pull over some place to rest for a few hours and then go back on the road.