It is not uncommon for the police to act as they did. They have authority to act on your behalf, but they often refuse when the "victim" has their own legal recourse and the matter is not of great public "interest" such as a property dispute between family members. There is really not much you can do to force them to take action. Your time and effort is best spent going after the wrongdoer.
Yes, you can sue your mother-in-law for conversion and fraud in small claims court seeking the return of your vehicle. You should add the new owners as defendants. I would also report the travel trailer as stolen with the DMV
so that a new title can't be issued to the "buyers".
Please let me know if you have follow-up questions.
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