I found that 13.5k seems to work great for 22' or smaller. I don't even know why an 11k unit is even still offered on any Airstream frankly (minus that T@B looking model they had at one time).
Back in 2003 I had several discussions about the 15k (both here and with Airstream) and was told here by a fair number of folks it would not work as it should, bad idea, blah, blah, blah, yet Airstream had the 15k unit as an option on 25' classics (not any other model back then).
I questioned that thinking when I ordered my 25' Safari since 25' is 25' regardless of model. Airstream agreed to install the 15k for an up-charge. Contrary to the many naysayers, I have found the 15k BTU unit an excellent performing appliance for a 25' Airstream. It does not cycle as much as a fair number of self proclaimed experts here were adamant about. I have no humidity spike as was suggested by a few here, and frankly, found afterward, Airstream started installing 15k units in far more models. Here I am nearly 9 years later and have not regretted my decision to go 15k-- ever. I also use the heat pump feature on my 15k (might also be avail on the 13.5k) when it's in the 50s to save on LP since the furnaces eat LP like a drunken sailor in a port of call with a fist full of fifties.....
Bottom line, I think if you are at 25' or larger, 15k should be mandatory not optional. Those who have been in my Safari in the full throws of summer, with direct sunlight hitting it, will attest that I could prob hang meat in the Safari. Less than 25, I'd say 13.5k would be fine.
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