Vulkem and alternatives.
When I was initially searching for Vulkem, I tried all the home stores around Houston, TX and all I found was siliconized gutter sealants, a no-no on Airstream trailers. I could not even find the right Vulkem in the commercial 18 wheeler parts houses. I ended up having to mail order a couple of tubes. What I did find retail at an RV Superstore (PPL MotorHomes), is Sikaflex and Par-Bond. Both of these are acceptable to use on gaps, vents, around window frames and panels and are approved by Airstream for such.
Others have found that Alcoa Gutter seal is an excellent choice for sealing shell panels as it "wicks" into the tight space between the overlayment gaps. I have no mailorder source for this. It is usually purchased locally due to requiring a hazardous material permit to ship.
-"You want to make it two inches - or, if you're working in centimeters, make sure it's enough centimeters for two inches."-Red Green