Hi 56mikeevers,
Welcome to the Heartland Owners Forum.
If you look at the A/C vents in the ceiling, you'll notice that one side has dark material behind the plastic louvers. Those are the filters.
Take a look at our owner-written
Landmark 365 User Guide. Page 30 has more detail on how to remove the covers to get to the filters. And of course there's a lot of other useful information in the guide.