I should add that the old 139s were insulated, but not very well. I assume that your unit has an aluminum skin, right? If so, and if you undo a couple of the screws, you will see ordinary pink fiberglass stuffed into the spaces between the studs.
By contrast, the newer X-139s have thick styrofoam bonded to a fiberglass skin. This has a much higher R value. Having had both types of trailers, I can tell you that the newer ones are much warmer.
Having said that, let's not lose sight of the fact that you now own a rare commodity -- the old 139s were surprisingly sturdy. So are the fiberglass units, but they are not nearly as "retro cool" as the oldies. I'm assuming that you got a smokin' hot deal on the trailer!
And note that there are almost no comparable models currently being manufactured anywhere, by any company. This is unfortunate -- what am I going to do when (not "if") I wear out my 2012 X-139??
Anyway, welcome to the club, and keep asking questions!!
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)