Something I would try while it's cold: warm the interior, and then using an infrared thermometer (looks like a gun, shoots a laser pointer at a spot and takes its temperature. About $25-35 at Amazon) Theoretically, a place that is wet won't insulate as well - so you should be able to simply stand in one spot and take temps at various spots in the RV. A reading that's a few degrees colder than anyplace else needs to be investigated. Granted, it might just be a void in the wall - but the IR gun is a cheap way to check insulation.