[bstark]

Geoff, I agree. the furnace draws air via the pocket door cavity and bypass leakage around the storage doors and indeed through any orifice cut into the belly material as well as back down through any gaps in the flooring such as that for the drain pipes under the sinks or wiring holes into center islands etc.. I find it difficult to believe that a furnace over-heat condition might exist due to a filter material being installed behind that grill above the pantry.
I am also of the opinion however, after trying the material in that location, while it does get dirty over time, since it is not filtering all of the air supply to the furnace, it is kind of redundant in nature.
I would think a furnace overheat would be more likely to occurr due to blockages of those flexible plastic heating duct runs which would impair the volumn of air being blown across/around the heat exchanger.
A blocked intake/exhaust combustion air port (small round ones on the outside of the coach) should be alarmed via the "sail switch" failure to allow ignition sequence completion.