The problem is that where you are thinking of going, with any type of coach, could get you into trouble. Once you get off designated roads, what is around the next bend is hard to guess. You just may find a large low branch that will not allow you to go forward, on a road that does not allow you to turn around.
Also keep in mind that most Emergency Road Services, like Good Sams and CoachNet, will not come to you if you are not in a designated RV park(either in the park or one of the park's approach roads), or are more than a hundred feet or so off of a paved road. Basically if you have a problem out in the sticks, you may be left to your own devices and may experience a big hit to your wallet.
They do make four wheel drive, off road type RV's that are not cheap but may be very well suited for what you want to do.
Off road RV's