75 lbs is actually a lot of weight to put on a frame mounted hitch behind your Airstream simply because the Airstream frames alone
are not designed to handle this. The admittedly overpriced Airstream approved frame (made by Fiama) uses the monocoque construction of your Airstream to handle the weight of the bike rack. Simply using the trailer's steel frame alone while not using the body of the trailer defeats the purpose of the monocoque construction and will lead to floor separation.
Before you go this route, I would respectfully suggest that you use the search tool and read up on the subject (this topic has been discussed ad nauseam on the forum.) Your "$500 savings" could eventually end up trashing your Airstream.