The OP did not indicate what model Montana he has, however the furnace in all of them seem to get some return air from the grates under the stairs. Since my 3402RL is the same as BigSkyJimmy's the vent in the storage compartment wall is another one of Keystone's engineering marvels. No one knows why it is there, it just is, and is hooked to nothing. As well the wall between the basement and the storage compartment does not go all the way to the top so again it begs the question "why is it there".
Here is a picture of the basement in my 3402RL looking from the storage compartment. There are three flexible pipes coming from the furnace, a 4" goes to bedroom duct, one 2" goes into the underbelly area and the other goes into a small round register in the toilet area. The living area in-floor registers are fed from the bottom of the furnace heat exchanger.
For us the bedroom with one register always seemed to be too warm, the shower area cold when closed off from the bedroom and living area, so I added a register nest to the shower that can be closed off and exchanged the one in the bedroom with one that can also be closed off. We now have a grand warm area when we get out of the shower on a cold morning and don't roast in the bedroom.
Simple mod made by cutting the floor the size of the new register into the bedroom ducting.
Closing off the 2" vent into the underbelly will not affect the operation of the furnace, you will get less heat into the underbelly area and your pipes could freeze if it is cold enough, that is the only thing you might want to consider.