[QUOTE=freebeard;422960]
Thermos bottles aren't 2-3" thick. That's 8-12" out of the width of your pop-up. What would you have for a source of heat?
Something very small and adjustable would heat the camper real easy. Like this one that adjust from 1600 btu to 3000 btu would do the trick ! I might only run 5 minutes twice per hour at 30 degrees outside :-)
Camco 57331 Olympian Wave 3 Catalytic Safety Heater camper Trailer RV | eBay
Tommy