I can't say that I have ever heard of Airstream using an aluminum frame under the trailer, but they did put an aluminum composite floor in some Argosy coaches during the 1970s. It has been confirmed by many owners that their '77-'79 Minuet 6.0 Metres have these composite floors as well as several of the 6.7 and at least one 7.3 Metre Minuet as well. One Argosy 20 has also been reported to have this aluminum composite flooring as well.
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)