Foldbak ... The youngest Montana that I've been in behind the rear wall in the pass thru storage area is a 2019, but I'd guess newer ones are very similar. The area behind the steps at the junction of the drop frame and upper frame is really all one big area connected directly to the underbelly. This link will show more so what I'm trying to describe.
https://www.montanaowners.com/forums...ht=design+flaw
On windy days until I did all my mods, you could feel warm air puffing into the living area right out the stair vents which
"are not connected via any duct work to your furnace".
PS Like said, plug up any openings in your underbelly as well.