Noticing the lack of insulating properties in my 1964
Globetrotter, I purchased 24 cans of spray insulation. I drilled holes through the inner skin inside the cabinets and removed light fixtures and recepticle plates to get additional access to the space between the inner and outer walls. I sprayed the insulation until the can was empty and then sealed the hole with a plastic plug. The plugs pop in place. They are available from hardware stores in the automotive specialty hardware trays. After spraying, you can determine from the temperature drop of the skin how far the foam has gone. Mark it with a pencil and you know where the next can should go.
The insulation has made quite a difference in the roof especially on sunny days. I used the "large gap" variety of the foam.
To do the whole Airstream Globetrotter would probably take 50 cans or a professional application kit. Good luck.