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Although some may not, I totally agree with ChuckS. There are so many openings for the furnace to source return air in any Montana model besides the "grills" beneath the steps that it almost makes the "grills" more of a suggestion than a function. Daryl mentioned several of those openings. On my 3150RL, there is a huge grill right under the fridge that makes my step "grills" more of a trim option than a return air source. Our RV furnaces are notoriously inefficient to start with. Also, unlike our stick houses, there is no dedicated ducting to supply return air ... the furnace just sits in an available location per floor plan drawing cabin return air at will. Like mentioned earlier ... the combustion air is totally separate with combustion supply and exhaust air sourced via the grill on the outside of your RV. Doing all the plugging of holes like Daryl mentioned will cut down on drafts, but the propane furnace will easily run safely either way.
Secondwind ... yes with any Montana floor plan you are heating "some" outside air due to the dozens of opportunities for air leaks as Daryl pointed out. We all are ... like you, many use supplemental ceramic heaters.
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