If the outside furnace space looks fine then what about inside the coach? If you can get visual access to the furnace case/body inside the coach, check to make sure no wires or hoses have drooped enough to touch any hot surfaces of the furnace (I “assume” the metal furnace case may still get hot enough near the combustion chamber to melt plastics touching it). The smoke detectors going off would seem to indicate internal smoke products being generated from something.
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