Hi Tom and welcome to the Forum!
We've always used a single rear wheel F350 for our 2955RL Montana and it is more than adequate. However, this weekend the wife and I were just discussing if we were to upgrade to a longer Montana (saw 2 or 3 new ones on the road), would we consider going to a dually. I would hesitate mainly because I would not want to drive it daily, park it, and most likely would loose my spot in the garage.
We concluded that if it came to the point we wanted more space than our 2955RL, we would sell both the TV and trailer and look at a diesel pusher!