In my mind it's fairly clear that coming to an open internet forum and asking about who is legally responsible, is likely a fruitless search for accurate info. The correct folks to ask is the insurance agents for both you and the trailer owner.
Having said that, if the question were a straight out, "what would you do in this situation ?".......then I would answer, I would suggest to her ( trailer owner ) take it to the shop of your choice, get it fixed, and I will write a check for the full amount.
That way there is no risk of hard feelings if it turns out the insurance company raises rates, etc.
My feeling is, when I am driving the vehicle, it's on me, 100% to ensure this thing moves without getting damaged.