The thing about RVs is that generally, you won't have to drive them often. Average solar panel produces about 15 watts per square foot, so over a course of a month with about 7 hours of sun a day, that is 3 kwh/square foot. So 50 square feet of panels, which is the total area of a large bed, will get you half the range over a month.

Also, there will undoubtably be hybrid options where you can have a multi fuel generator charging your batteries.