I don't recall seeing them on an Airstream, although there is a Spartan in our shop with fender skirts. They are constructed with the same aluminum sheets as the skin of the trailer, with mounts riveted tot he back of them, and a hook-shaped hinge/mount at the bottom. There is a locking latch with a lever to keep the skirt attached at the top. The ones at the shop are buried somewhere in the warehouse, I'll try to take a couple of photos if I can find them.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.