There is no difference in the glue between exterior grade fir and marine grade plywood. The only difference is marine plywood is sanded 2 sides and has no voids in the inner plys. I seriously doubt that Airstream ever used marine plywood for anything in the catching industry we used exterior ply in the inner parta(the soule "floor" for example.
Shellac is possibly the worse sealer that you can find.If you relay want to seal off the ply use Epoxy penetrating resin and double seal the edges as capillary action will attract moisture and start the rot.
Jim Smith
1965 Tradewind(The Silver Abaltross)