Here's an old post of mine ... maybe not titled well ... "design flaw", but my post addresses a continuing issue with the dropped framed Montana's. The pass thru storage is by no means closed off from the exterior weather. The underbelly isn't air tight, and the gap between the underbelly and rear storage wall (once the wall is removed for access) is good sized. Some people have put thick Styrofoam in-between the floor beams as best they could, but like Bob mentioned, you'll still have very conductive aluminum beams. Without plugging up the good sized gaps as seen in my old post ... you likely won't gain a whole lot. The front storage area beneath the front closet isn't any better as far as sealed to the weather. Lotsa gap around the landing gear.
https://www.montanaowners.com/forums...ht=Design+Flaw