Re: Trailer Reg in California
I have titled several Special Construction vehicles in CA including a trailer. This was a few years ago but this is how it used to work and probably still does.
You need a receipt for an unregistered vehicle (trailer) or receipts for the parts if you built it yourself. Tell DMV that you need to register a Special Construction trailer. DMV should review the receipt and then give you a referral to CHP. You call CHP and find out who your nearest Inspection Officer is and then call him and make an appointment. You take the trailer to the appointment, he takes the paper from DMV, inspects the trailer (or more commonly doesn't really), attaches a Special Construction VIN sticker to the trailer and gives you a piece of paper for DMV. You go back to DMV with the paper from CHP and they title the trailer.
It rarely goes this smoothly and sometimes takes more than one visit to connect with the Inspection Officer. I got the impression that it was a chore that they would rather not be bothered with.