Welcome to the forum! Airstream has historically limited the permissible weight for a rear bike rack to 75 lbs. +/-. More weight than that exceeds the design parameters, and can lead to dangerous sway issues.
The new search function above works great [inside the blue box] so I would encourage you to search for:
-- rear hitch
-- roll over sway
-- Fiamma bike rack
-- etc.
Notable among the many threads documenting sway accidents are these:
http://www.airforums.com/forums/f48/...ec-153984.html
http://www.airforums.com/forums/f238...ay-156615.html
http://www.airforums.com/forums/f238...ay-152451.html
The newer Airstreams may permit heavier weights on the rear, but nothing has surfaced here to that effect IMO. The owners manuals are available online at Airstream.com. [See PS below]
Good luck,
Peter
PS -- Here is the 2018 Classic Owner's Manual:
https://www.airstream.com/wp-content...ers-Manual.pdf
In the limited edit time for this post, I don't see anything on a rear hitch or bike rack info.
PS2 -- AS approved Fiamma bike rack:
https://store.airstream.com/product/...kes-by-fiamma/