Actually I was serious.... This is an oft discussed question with those redoing molded fiberglass trailers that are exactly the same construction as your Sunrader. Anything you can "Spray" in will have a low insulation value, R-3 if you are lucky, will be a mess and very difficult to cover. Covering the windows
with removable "Refletix" (sp?) helps, but real insulation needs airspace, such as double pane windows
and an inner hull.
Unless you are wintering over on the northern tundra, my earlier suggestions are what works, and what you will still need after anything you can spray in place.
The later BigFoot
trailers from Canada are available with a "All Seasons" insulation package that adds not only substantial cost but substantial weight
"Some stuff' and "Some sort of backing" just won't get you very far.