My biggest concern would be the engineering that would have to go into moving the rear axle. Most trailers, the axles are about mid-way back the trailer. On a motorhome, I envision them much closer to the back. If you don't move them forward, the pin/hitch weight on the truck is going to be quite high. If you do move them, then you're looking at some incredible feats to handle the modifications to the frame and whatnot. I don't see how you're going to do that without having to hack up the inside.
What's your budget on this?