We are hearing more and more about Airstreamers using or being interested in using foam insulation for their Airstream restorations. While it could be a great option, if applied improperly, can be quite a disaster. Also keep in mind future repairs. To get to the area to repair you have to dig out the foam insulation, and most likely refill it with regular insulation.
The pictures below are of a vintage Airstream with the foam insulation. The aluminum panels actually warped probably due to the over-filling with foam and there being not enough room for expansion when the temperature changed.
Moral of the story, if you choose to go with foam make sure you choose a professional with experience and understanding of these possible challenges.