It's been several years ago, but after I tried to register a trailer at two different California DMV offices and turned down by both, I went to the Triple A office (AAA) here in Long Beach, CA.
It took them about ten minutes to register it. They have a hook-up with the state and I had my paper work, plates, etc. I asked them why they could do it and the state couldn't and their answer was everyone working in the California DMV weren't aware of the rules. They showed me in the State book where it could be registered, etc.
I paid my fee and had everything in my hands withing ten minutes.
If your an AAA member this service is part of your membership fee.
Good luck -- Don