Blocking the vents under the stairs doesn't do awhole lot in the whole scheme of things ... at least on my 3150 as there's way more places for air from the storage and belly to pour in. Here's my old post ... it really really works for both summer and winter. This is mostly for units with the dropped belly frame. When you remove the rear wall in the storage compartment, you can look right down into the belly between the aluminum wall studs and see light usually as the belly pan is by no means air tight. Also, on most any Montana fiver if you were to sit inside the storage area with the doors closed ... you'd see that the door seals seldom seal all the way around as the frames aren't square.
http://www.montanaowners.com/forums/...rms=insulation