We are "semi-boondocking" on a 20 amp single plug and therefore running the refrigerator on propane. Have not noticed this overheating of cabinets in the area. There is a chimney built to the roof, to allow proper venting of exhaust fumes from the propane side of the frig. I'd check to make sure that is open and getting good draft. If you go up on the roof, you will find this long vent cover right over the refrigerator. It is removable with two screws, that are usually under caulking. Pop off the caulking and uncover the screws. Remove them and the vent cover lifts right off. Gives you a clear view and path down to the back of the frig.
By the way, this is a great place to route a wire from in the cabin to the roof. I've used it for solar panels and power sources for getting 12 volt to the roof on other RV's.
Wonder if these are old soot marks? I can only imagine the venting not working if the vent was covered and the propane frig was running.
Ric and Jan Golding
2016 Jayco Seneca 37FS