I check mine with a flashlight periodically and clean them in the sink. I was told also to insure the return air vents point towards the closest outside wall.
I don't think there are any filters on the roof unit itself, but it is on my list now to ask next month at my factory service appointment.
I saw some newer coaches at the Southwest Regional Rally in April that only had ducted outflow outlets. The return air vent was under the roof air unit.
Dan & Sharon & Kasey (Our Yorkie Puppy (12 Yrs Old) On the Road (2012 Journey 36M, 2006 Jeep Liberty)
USN-Ret ('65-'93) Fulltimers, Class of 2012