Dometic DM-2652-RBFX Works w/AC Not w/gas

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Our refer (in a 2011 Flagstaff) has stopped cooling when using propane. It works quite well when on shore power. Our normal mode of operation is using propane so the majority of the 150+ days of operation have been with gas. Here's my observations/symptoms:

1. When inspecting the burner assembly I noticed some black/rusty dust and flakes on the burner that had fallen out of the flue. The RV is almost exclusively used and stored in the mountains of New Mexico so I have a hard time believing that the flue is rusting out. Perhaps this is soot build up?

2. I gave the flue a few raps with a screwdriver and a little more of the stuff fell out of it. I then cleaned all of this of the burner then reapplied power and gas. The burner lit right up and it has a nice blue well formed flame.

3. The fan up by the condenser cycles on and off (normal) when the refer is running on shore power but not when it is on propane (even though the burner is lit). The schematic (attached) seems to indicate that there are 2 thermofuses (one for AC and one for gas?) but only one of them is shown as wired through the fan.

4. Has anyone ever used something like a shotgun bore brush to clean the flue? I am worried about damaging the flue baffle that dangles inside the flue.

5. I didn't know that the "thing" on the flue was a thermofuse until reading through posts on this forum (thanks for the picture post FRSalem2006) so I don't know if it resettable nor did I run a continuity check on it. I'll do that when I go back to the trailer (3 hours away).