I took the approach of the whole chimney. Pulled off plastic cap roof and was amazed at how much stuff was caught just under cap and under the screen. Didn't look like much until I looked down with a flash light. Between a un-bent hanger (to get under the screen) and a leaf blower it is now clean top to bottom. I also "scotch brite" pad the contact points and gave the chimney a couple of gentle swats with hand to get junk out.
I really don't think there is any danger of fire. If there was we would hear about people melting their plastic roof cap, but wonder about efficiency of chimney with it being somewhat blocked. My neighbors trees don't help.
Anyway and after cleaning it out and cleaning contact points runs 100% better. There is also some youtubes on cleaning and giving it a "tune up".
Also picked up the plastic coupler thingies that keeps frig access panel against RV, just in case one of the two little plastic knobs broke. There is not a lot keeping them on.