Bruce....Alot of the coach manufacturers say that you shouldn't even remove your air cleaner to inspect it. They say under light use they could be good up to five years. You are better off to rely on your gauge. Check it regularly and reset it when it gets to the red zone. If it goes red again, it time for a replacement. They say a hard pull on a mountain grade can cause them to go to red. Just reset and check later.
I pulled my air cleaner out, inspected and shook it before I started reading all of the articles to not even open the air cleaner canister until your ready to replace.
You might want to tell service guys to leave it alone when it's being serviced, unless you do your own.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab