Tom... Let me just ask you why you were towing their new trailer with your truck. Not saying it changes any legal obligations, but in my opinion it makes a difference.
1) had you borrowed it and they were guiding you back to the storage area?
2) owners truck broke down and you came to the rescue 200 miles away to tow their trailer home as a favor because they didn't have towing coverage?
Again I'm not passing judgement, but in my mind, the big picture sets the scene for the moral responsibility