Not only is this poor design, but many of the rub rail rivets go thru the base channel into the floor. Airstream's placement of rivets in this area are the source of leaks and rotten floors. Often theses leaks can only be found by removing the inner skin.I have used the rubber roofing fire and ice membrane sliding it under the side skin and over the belly pan. Once I reinstall the rub rali. I use a razor knife to remove any excess sticking out under the rub rail.
Lots of frustration , cussing and leak testing with the garden hose.