Wow, Hopefully that is just dirt but that looks like a carbon buildup from using propane to heat (if possible), that or something is burning. That raises a good question, how do the Atwood heaters vent when heating with propane? I've only used the electric heater thus far.
Wayne, Kim, Mallory & Abby the Dog
2013 - Jayco Jayfeather - X20E
2012 - Toyota Tacoma DBLCB Sr5 with towing package
Equalizer 10k WD, Prodigy P3 Controller, TST TPMS
2012 - Jayco Sport 10 Popup - Traded