I found this info on the Thor site:
Thor Industries, Inc., was founded on August 29, 1980, when Wade F. B. Thompson and Peter B. Orthwein acquired Airstream, the most recognized name in the industry. The name "Thor" combined the first two letters of each entrepreneur's name. Despite its famous image, Airstream had not fared well during the economic downturn of the late 1970's. By focusing on improving quality while reducing costs, Airstream returned to profitability in its first year under the new Thor management.
I have also looked for 1980 model trailers and there seems to be few for sale. I think the production runs were down due to the company being in poor shape. 1981 models are out there but not in the numbers on the 77,78,79 and 82 and up.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato