I am very sorry for your loss. You sound like a very caring person and it sounds like you did everything you could to prevent this from happening.
I agree with you completely about the poison. I have several friends who loss their pets due to the accidental injestion of rat posion. Another thing people don't realize when they use this product is that animals can also die by consuming the rat who ingested the poison. There have been several suggestions to companies about this product, such as making dyes as an indictactor that something had been ingested. But I am not sure if any of these have been followed.
It seems like the person who sold the trailer should have some responsibility for what happened. The 'Right' thing for them to do, would be to pay the vet bills and the cost of the dog, but I am not sure if that is a legal thing. There is a site that answers pet legal issues,
http://www.answerbag.com/c_view.php/1819 (not sure if these questions are answered by experts) another more technical website is
http://www.nabr.org/animallaw/Damages/index.htm. If you went to court, it would have to be small claims since it wasn't intentional, and you might have to pay court costs, but if you go to the person who sold you the trailer, with the info and explained you had a case, maybe they would do the right thing and compensate you. Its the least they could do.
Good luck to you and your family, Michelle