There is indeed one use for silicone sealer on the exterior of an Airstream.
Installing the opening windows on a 66 Airstream, requires the use of a sealant within the hinge, to hold the glass in place.
That's how Airstream installed them.
In 67 and 68 the window hinge opens up allowing double sided tape to be used.
We agree, that Vulkem would be better, but it takes many hours to set up. Next, if that same window ever had to be replaced, removing the broken glass and the old Vulkem sealer, would be a task that would most likely generate more "nasty words".