Gordon, as a California resident who did have a 1997 21T that was not properly registered when I bought it, I would think you need to make an appointment with DMV and take the trailer in for proper registration. If it currently has California license plates, DMV can run the tags and tell you how it is registered in the system. Be prepared to pay taxes on the purchase price, which is what ever you tell them. Any travel trailer with a length of 21 feet has to have license renewal every year.

Good luck,
Garry