On our trip cross country last summer, our fridge
wasn't cooling well at all when we got to the Grand Canyon and I first tried to blow from bottom up the chimney and quite a bit of soot came out and fridge worked a bit better. The next day I removed the top grill and found that there was actually a cleaning rod down in the chimney that I had never used. I pulled this rod up and down several times and it screws its way along the wall of the chimney and clears the soot.
Fridge worked the remaining 2 months of the trip with no problems on Gas.