As you do not really need all the features of an RV, what about looking for a small touring bus with a bathroom? They generally have comfortable seating, a toilet or you could add one, and I am sure you could partition off some of the space to give your son a quiet room.
Most RV bathrooms are tiny and would not accommodate an adult and teen.
Something to think about with RV seating. Many people get car sick if they are not looking forward. If you do choose and RV you may want to rent and see how the various family members do if they are looking sideways or backwards before putting down $$$.
