Was the hitch installed at the factory or by the dealer? Did U-Haul use the 7-pin or 4-pin plug (I would think 4-pin). You can purchase a tester for either from Amazon to test it yourself:
Based on a lot of experience with U-Haul and the neanderthals they seem to attract and hire, I would believe the problem was with their trailer - not your GC. I have had this happen several times at U-Haul myself, and ended up showing the U-Haul mechanic the problems with their equipment and how to fix it.
U-Haul equipment is typically poorly maintained at best - tires, wiring, bad bulbs, doors, ramps - many issues. I check their crap over front to back and document everything wrong, or they will try to charge you for it upon the return. Unfortunately, they are often the only game in town - particularly if you want to pick up and drop off many miles away.