I haven't had the pleasure of taking one of these guys apart, yet, but I would bet the flame sensor along with the sail switch is inside the furnace proper. Sail switches cause problems too so might as well check it too.
Have you checked the flue for blockage? Might be mud dauber's nest or wasp's in there if you have them in the GWN.
2013 Voltage 3800, 2012 Chevy 3500 HD
2010 Yamaha V Star 950
2009 Yamaha Raider
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