Butyl bedding tape is also available at most RV supply houses, HVAC supply houses and metal/roofing suppliers. There are some minor differences in the formulation but shouldn't make any difference in this application. It also comes in varying widths and thicknesses. I have a couple of rolls floating around here that are 3" wide

But I think on the Airstream the best stuff would be the 3/4"x1/8" or the closest to that you can get. BTW real fun stuff to work with on a 100+ degee day
Aaron