It should be 12" brakes on your trailer. Just go to almost any trailer repair shop, and pick them up. They are standard springs that have been used for decades on almost every trailer with electric brakes. They are not Airstream-specific.
Now, if they are really old brakes, I would strongly recommend replacing them with loaded backing plates. They will come with shoes, springs, magnets, and adjusters, and are pretty much bolt-on. SHoes don't last forever, no matter how thick the friction material is. Eventually the epoxy that holds the friction material to the table will fail, and most likely will cause the brakes on that wheel to lock.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.