I have a contract with a person who was leasing my travel trailer and they have disappeared with the trailer, what is the SOL in Texas to file a criminal suit? The trailer was over 18K.

You don't file a criminal suit - the Police take care of the criminal end of things. You file for damages, a civil matter.

This is NOT a Small Claims case so you will need an Attorney. Of course, if you can't locate the person that will take extra time and money.