We purchased our TT 3 years ago in California. We got the title from the seller and she signed it. It looks like we were supposed to have turned it into the DMV within 10 days, but we were crazy busy ...
Apparently you never licensed the trailer after you bought it. You should be able to take the title you have to the DMV office, fill out the paperwork, pay the fees and get a new title. There should be no need to get California involved.