It's not all on U-Haul. Once hooked up, whether you or the U-Haul rep does it, check everything. If necessary, shorten up the chains by twisting them, which is totally legit. If the chains are just slack enough to turn sharply, and crossed under the tongue, they should hold a loose trailer reasonably well. The customer (renter) needs to use a bit of their own diligence in something that could risk their own vehicle, besides the trailer.