Actually we discussed thermal breaks quite extensively and have gone to the trouble of covering all of the steel with 1 inch of insulation. We were able to do this by creative placement of the 1x1 and 2x1 steel frame members. Right now in our garage, with just sheet plastic over the door and window openings we are able to keep the inside at 80 degrees with a small space heater while the garage is 30. For a truck camper, I think we managed decent insulation...if we could be the mass of your fifth wheel I might have gone for three inches.
