Be aware there is a baffle of sorts INSIDE the flue, a corkscrew looking device used to control heat dispersal for most efficient operation - if that gets dislodged or damaged, you'll end up with far worse problems than soot in your flue...
Fact is, unless your propane burner flame is seriously misadjusted, the flue probably won't need cleaning - ones I have seen after over 20 years use sure didn't need cleaning...
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John Day....|'88 Winnebago Super Chief 27ft. Class A
Eastern .....|'88 KIT model 240 24 ft. 5er
Oregon ......|'02 Dodge/Cummins 2500 Quad Cab