If he was supposed to get the trailer, why haven't you signed it over? Contact your divorce attorney and tell him or her you need to get the trailer out of your name. You may want to keep the trailer in order to recover some of what he stole from you, but the potential liability of having this trailer in the possession of his (apparently irresponsible) family is not worth the risk.
That is just my opinion based on the facts you describe. Your divorce lawyer can help you with more precise information.