Is it the flow of cold air around the exterior service panel that is the problem?If that panel isn't secured tightly, you could either reseal it with butyl tape or caulk around the perimeter with a good sealant. I like the Geocel sealants like ProflexRV.
As others have remarked, the combustion air intake and exhaust are in a contained space with the burner chamber. THere should be no cold air from the exterior exiting that shared space.
Using the plumbing system in a Montana Winter will require the furnace to run almost continually and preparations will have to be made for keeping the waste tanks and drains unfrozen. This usually involves heat tapes, tank heaters, and added insulation.
