The U-Haul trucks I've rented in the past 30 years have always been pretty beat up. They may have been serviced properly, but the inexperienced drivers had trashed them physically and mechanically.
In addition, the trucks were sparsely equipped with very few creature comforts and the least powerful engine and transmission combination.
If you are looking at converting one to an RV or Tow Vehicle, the cost of conversion would be a lot more than buying a used motorhome or a Pickup/MDT already setup as a tow vehicle. You'll get a lot more for your money in the long run buying a good used RV/Tow Vehicle.
www.uhaul.com/trucksales