Like said, the vents are supposedly for the return air from your living area to your propane furnace. Don't be fooled in to thinking this is a closed duct behind the vents to your furnace as it is far from that. It can draw air from your belly (which is not completely sealed to the outside air either), storage compartment, and the living area itself. That is why you feel hot air from the vents in summer and cold in winter. On my 3150 ... I could eliminate the vents and it wouldn't make a bit of difference as there is also a large wooden grill under my fridge that is connected to the area that your furnace draws return air from plus all the ways to draw supply air from mentioned above. I'm not saying permanently block your vents as you do not have my floorplan, but I am saying return air comes from a lot more places than just those step vents. I have provided the following link to an old post of mine a few times before, but I'll do it again. I doubt if for many of the floorplans with the dropped frame that there has been a design change to the later models.
http://www.montanaowners.com/forums/...ht=design+flaw