I think those trucks are a great idea to base one on. I myself am builing a Ford F-700 using a 24 foot uhaul box. Although I am gonna use ramps for my Jeep Wrangler.
There are many people using the uhaul trucks for this. Of course the uhauls as are most MDTs are not geared great for open highways and/or decent MPGs.
There are many threads at
Pirate4x4.Com - The largest off roading website in the world. look in the towing section. Also all the uhauls trucks I have seen for sale lately are gassers and max out at about 18K GVWR.