||Having an "81" GMC, Thomas built, propane conversion, I soon found the head room is a problem(I'm 6 ft ). I opted for the spray insulation on the underside and inbetween the 1/2 round moulding on the exterior. As well as copious poly stuffed glued and caulked everywhere else you could think of. For the past 3 winters I've had ample time to narrow down numerous drafts. the front compartment (the steel frame wall runs directly behind the drivers seat)is fitted with interlocking panels made from regular tarp material into long flat bags stuffed with, you got it, bits and pieces of all the poly I've saved over the years. Closed cell foam sleeping pads are excelent for window covers, you just have to hope and pray to find them in number at a low price.
More tricks developed daily :-)