Never thought I would suggest that someone not mount something on the rear of their Airstream. But in your case I think a tire mounted to the Older Small Frame is going beyond the trailers limits. Even if you bumper would support the weight bolting through the body skin to take the rotational moment of the mount would tear the skin in short open order. Even if you put a plate on the inside of the trailer and caught 2 of the vertical structural members I would have reservations.
Where did you get that mount and what lead you to believe it was to be mounted on an Airstream?
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles