If he is just a mechanic at Trinity and you talked to him or did this deal when he was working and have some paperwork or proof, I would be on the phone with trinity's owner in a heartbeat. Don't get emotions involved, just tell them what happened, what the deal was and what has happened. Also ask them if they would be willing to make their employees screw-up right. Phone calls cost nothing and may get you something out of it. Worst they can tell you is no and I am sure you have heard that before in your life. Sorry to hear you got the screw but anytime somebody does something on the side it can very easily happen. If I owned Trinity and you had some kind of documentation IE bill of sale on the chassis etc I would be pissed at the mechanic taking money out of the companies pocket.
Good Luck.