Certified RV technician
A tool I have used many times over the years is the flue brush. They come in different sizes and lengths; but as there are a number of different sizes of flues and vents in RV appliances, having a set that will fit them all has been good.
The odorant used in natural gas and propane is ethyl mercaptan — spiders like the smell of it and gravitate towards it to nest and make webs. The little, fine webs they make cause LP appliances to malfunction. Mud daubers also have been known to make their muddy little homes in tubes and flues in RV appliances. The most thorough way to clean out some of these nests is with a flue brush. Small flue brushes can be found wherever appliance and plumbing tools are sold, or from Amazon.