Dario - Did your repair and maintenance manual give you any ideas on what is meant by "frequency of use, quality of propane and region of country."? For example, the house refrigerator coils probably should be cleaned every other year - It runs continuous. It likely means wait a couple seasons, check it and if clean wait a couple more and then check. You will learn how often to clean based on your style. All I know for sure is on my previous 1977, the flue was never cleaned by me and the refrigerator still worked great.
Just wondering?
Gene
2002 Trail-Lite B Plus 211 S w/5.7 Chevy