The "chimney" is the open space that runs from the bottom of the fridge up to the upper vent area in your slide out. If you remove the exterior upper vent cover above the fridge at the top of the slide out you need to check that there is a baffle installed that directs the hot air out the vent. This baffle may be nothing more than a piece of paneling that keeps the air from circulating back onto the top of the fridge. If you have the installation manual, there will be a diagram of what I'm not having a successful time describing. A Google search will also yield the manual. How do I know this ? My Atwood was exhibiting the same symptoms as yours. When I pulled off the upper vent, no baffle. I made one out if luan plywood and added additional fiberglass insulation in the "chimney" area and all is good.