Thanks to all for their responses and opinions. I do see great potential and originality in this conversion. I have owned other motorhomes, and I actually like the driveability of this old Uhaul over my last motorhome. I want to maximize R-value.....and with this Uhaul conversion, I could have 4 inch thick insulated outer walls, and even more in the cieling. Also tossing around the idea of "doulble" RV windows......using one for the outside, and one directly on top, but on the inner walls of the box. Perhaps use 2-3 layers of 1" thick blueboard under the floor. I like the idea of heating/cooling very effeciently.
So, I am still in the thought/planning stages now....but see this as a fun project. Wish me luck.