Hello from Colorado. Welcome to the Airstream community. You jumped in the deep end with your
Sovereign and full timing. Maybe you are planning on getting out of the cold Great Lakes area and heading south.
You can research the "Wally Byam Caravan Club International" or WBCCI, and see if they have any records on that number. That 5 digit number seems high to me for a club number. My 75 Overlander 27' has a four digit number beginning with 8XXX. I plan on joining WBCCI and hope I can keep this number. You can search these Forums for the thread that researches these numbers.
My Overlander had quite a bit of rust on the rear crossmember and rot on the plywood subfloor in the rear. The body became detached from the frame back there. This is called "rear end separation". My frame rails were sagging about 3/8". It is a rare 70s trailer that does not have this condition unless it has been repaired. I'm repairing mine this winter. This condition is caused by a Airstream design flaw involving a poor rear body to frame water seal and I think too much weight on the rear of the trailer. Maybe you did the "bumper jump" test before you bought your trailer. It is a necessary and yet not terribly expensive repair to keep thee vintage Airstreams strong.
Jump right in these Forums. You can learn about every rivet in your trailer, learn about nice places to visit, have a chuckle or two and make some new "pen pals".
David