Airstream decided a number of years ago to discontinue making replacement "double pane" windows, because of the small demand.
They now have discontinued all "clear glass" windows, because of production costs. A very short supply of a few clear windows, is still available, but cost much more than the solar gray or bonded glass..
In the future, "solar gray" or bonded glass windows must be used as replacements. The metal frames on the fixed windows are not exactly the same as the old style, but they will work. The dimensions are slightly different.