Hello, my name is ***** ***** I will be helping you today.
You basically have two options since the police won't get involved (this is a common practice by the police since they cannot tell who is the rightful owner of the car).
The first option, and the safer option, is to sue the person in small claims court to recover your trailer. The court will issue an order which states that the person has to give you the trailer and you can have a sheriff have this order implemented.
The second option is to go onto the person's property and take the trailer. If you are the rightful owner of the trailer, you cannot be convicted of theft (you cannot steal your own property). However, the person may be able to press charges against you for trespassing or breaching the peace. If they do not give you consent to go onto their property and get the trailer, most likely you could be found to breach the peace. If the trailer is on a public road, then it is less likely that you would be found to have breached the peace.
Simply because you owe them money, that does not give them the right to keep your trailer. They have to go through a legal process to put a lien on your trailer and have it sold if they want to recover money from your trailer. Know though that if you bring a lawsuit, they could sue you back and try to obtain the money they claim you owe them.