The bumper on an Airstream will hold the vertical load of bikes but will not restrain the rotational force about the point of connection.
The picture shows how I transferred that load to the window frame. The rack is attached tot he bumper with 4 bolts and wooden blocks to match the couture of the bumper. I made small clips to attach to the window frame to accept the straps.
It has worked for over 10 years and more than 50,000 miles. However I am seeing some sag in the yellow arms.
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