I have taken the top cover off my trailer and there is no way I can access the top of the chimney. There is a raised rectangular screened port that allows the heat out, but no access to the top of the chimney. I've had many old trailers, but have never had to clean the flue, and the only possible way to access it is to take the fridge out of the cabinet. I have tapped on the chimney, and gotten some light rust, nothing crazy, no big chunks or flakes, what I would consider normal.. And if there is a blockage, which I doubt, why would the external temp of the chimney, be the same as it is when it's plugged in?