Seems to me that insulation typically works one of two ways. It reflects heat or traps air without providing a conductive path for heat. Think those foil like survival blankets (reflective) or a down comforter (trapped air).
More inches of trapped air = greater insulataion R value. I would think a layer of insulating paint
can only provide as much trapped air as the paint
layer is thick. If the paint layer is as thick as say ensolite or reflectix it could possibly provide similiar insulation value, 1/4 as thick then only 1/4 the trapped air possible so only 1/4 the insulation.
Based on my own experience winter camping I can say that 3 inches of straw covered with a canvas tarp has kept me as warm in a light
sleeping bag as my very thick sub zero rated sleeping bag. Hey I figured the survival manuals all said to use dry grass or leaves to keep warm so I would test it while I had the sub zero bag handy.
It should not really require much insulation in these small campers, just enough to prevent temp difference between inside and outside from creating condensation on the walls.
The forum post above is from 2007 a later Scientific America article from 2009 pretty much shot them down as having any insulating capacity. http://www.scientificamerican.com/article.cfm?id=benefits-of-insulating-paint
The only thing that they say these paints can do is reduce heat gain in direct sunlight if applied to the Exterior
surface. Or reflect radiant heat back into a room. Does not sound like they would do much applied to a FG camper interior wall.