Justin,
I have had similar thoughts of using an old U-haul truck. My reasoning, twofold; A mobile garage (of sorts), and solves the problem of having to buy a seperate tow vehicle and enclosed trailer. I have been looking for one that is 20+ feet, so I could mount a work bench, cabinets, generator, etc... As with an enclosed trailer, it would also serve as a rolling billboard for sponsors. Of course, when I want to locate a used box van I can't find one for a reasonable price in my area. lol!
Steve