So far as I have been able to learn, little information related to your quest will be available from either Airstream or WBCCI even if there were numbers on your trailer. The one possibility that might be available to you would be a title search via your local DMV. In Illinois this was possible via the local currency exchanges for a modest fee - - the search would list all owners of record for a vehicle with a particular serial number - - I will admit that it has been several years since I accessed the service so can't relate current availability or cost (and it would most certainly vary by state). I don't know if this would be possible in California, but it could be. The limitation, at least in Illinois, was that the search would dead end at the point at which the vehicle was brought into the state - - in Illinois, I was given the last state of registration prior to being registered in Illinois.
Good luck with your search!
Kevin D. Allen
WBCCI (Lifetime Member)/VAC/Free Wheelers #6359
1964 Overlander International/1999 GMC K2500 Suburban (7400 VORTEC/4.11 Differentials)
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado Convertible (8.2 Liter V8/2.70 Final Drive)