I have no faith at all in NADA Airstream prices once you go back more than six or eight years. For example: Average Retail for a 1990 Excella 25' - $8,790. This being a dealer price, mind you, not a private exchange. What would one actually bring? $12,000? $15,000 maybe?
A big part of the problem is that they supposedly use reports from dealers in making these reports. Just how many 8, 10, 12 year old Airstreams actually pass through the hands of NADA reporting dealers each year? Very few, one suspects.
But, as you come to newer and newer units they may well become more reliable. Certainly your bank and insurance company are going to think so.