As part of 'going through' the truck I checked the various air filters.
I was surprised to find how dirty the main air filter was. I thought the Kenworth dealer might have changed it since they changed so much other stuff. The Fleetguard P/N is AF26163M. Prices vary greatly and Ebay was my friend.
The cabin air filters were filthy. I used Fleetguard AF26405.
The cabin air filters are very easy to replace as long as you know where they are. The first is the filter for the outside air. It is behind the air conditioning unit under the hood.
The other one is in the passenger side storage box and is easily reached by lifting the passenger-side access panel under the bunk, or dinette seat in my case. It filters the air used by the bunk heater/AC.