U-Bolts breaking during installation

covered wagon wrote:
Need double nuts too so they're locked tight and stay tight. That's important with temperature changes and vibration.

No, you don't and if temperature changes and "vibration" loosen your hitch receiver, you have other issues that a double nut won't cure.
(Double nut IS one solution when necessary, but when is the last time you saw an OE application of anything on a vehicle in a double nut configuration? Of course, there are many other, cleaner ways to accomplish the same thing.)
And has nothing to doo with the OP's issue.