The proper thing to do is make sure it's street legal. Lights, chains, proper axle with brakes depending on size then take it to chp or dmv for an inspection. They will blue tag vin stamp it and create a registration. Do NOT tow it to the inspection! Haul it on another trailer. Sounds stupid but they could totally fuck you and impound it for being on the road illegally.
I had this problem on an old trailer for my Whaler. I called the CHP office and they set up an appointment for me to bring the trailer to their station for inspection. It did have a license plate, but the registration was outdated and I had no title paperwork for it (I did have all he paperwork for the boat). They inspected it and then riveted an aluminum VIN replacement tag to the trailer. It was a very easy process.
If you tow it in the HOV lane alone it is automatically titled and registered.
The DMV doesn't VIN blue tag trailers anymore. CHP office does.
It must be a gray area of the law as I never see or hear of people being pulled over for it.
I've pulled my boat in the 91 express lanes, a half dozen times. Never a ticket or added charge to my account from 91expresslanes.
You can download the forms from the DMV website and if you call the Santa Ana CHP talk to their field VIN verifier (officer White... very cool guy). It's the last office that does field inspections. I live in the 55/91 area and he has come out twice to verify the '31 frame stamping AND affix the state VIN on the Canyon trailer. AAA can't do the trailer titling, but get ahold of a "registration service". They went to DMV and handled everything for $60 (including fees!).
I haven't done one for about 15 years, but when I did I took the trailer to DMV. They assigned a VIN onsite. Someone from the DMV went out and installed an etching sticker with the VIN to the tongue of the trailer. They gave me a plate and my paperwork and I was done.
I am going thru this as we speak. I was told by the registration guy I use, just tow it to the dmv. go around back and pull into the inspection lane, they will come out, inspect and issue a new vin then take the paperwork to a registration joint and hand them the paperwork and deal is done. NOW if you haul it to the chp with-out plate or paper work the fun begins unless it is on another trailer. I would cross your'e fingers and tow it to the dmv have it inspected and carry on. By the way the guy I use knows all the ins and outs of doing this. I have towed trailers with out plates many times and never been pulled over.
I'm telling you the DMV changed how they do this. Make an appointment at your local CHP office. They will put a VIN number on it. Then take the paperwork the officer fills out with the VIN info. He has to sign it. Then take the paperwork to the DMV. Done.
Ok. I'll call a different CHP office.
About 5 years ago I had a boat trailer out of Colorado without a title since they do not title trailers. No vin on the trailer either. I towed it to the DMV, they put a vin sticker on the trailer and I was on my way.