I can tell you that our 11k btu unit will freeze us out of our 25', but it's not an Airstream... especially not a '72 Airstream. The Armstrong unit I had on my '70 Safari 23' would freeze me out. The 13.5k unit I had on my 34' blew cold but couldn't begin to keep up on a hot day in the full sun. I had to run BOTH 13.5k units on our '85 325 moho just to keep some semblance of not being an oven on a hot day in the sun.
Yours should blow cold air, but whether or not it'll keep up with the heat in your trailer will depend on many factors. If you're concerned about its operation, I'd suggest you take it to a dealer and have it evaluated. IIRC, it's designed to cool 15 or 20* below ambient air temp or something like that.
AIR 2053 Current: 2006 Born Free 32 RQ Kodiak Chassis, & 1995 Coachmen B-van
Former Airstreams: 1953 Flying Cloud, 1957 Overlander, 1961 Bambi, 1970 Safari Special, 1978 Argosy Minuet, 1985 325 Moho, 1994 Limited 34' Two-door, 1994 B190 "B-Van"