I believe that the basement air/heat pump units themselves do not have filters. However, the coach manufacturer may choose to add them. On my 2003 Suncruiser both the furnace and basement air filters were underneath the bed, which is hinged. Maybe if you list what kind of RV you have others may be able to help out with more specific info.
Mark & Leann Quasius
2016 Cornerstone 45A
2007 Allegro Bus 42QRP (Sold)
2012 Jeep Wrangler Unlimited - Rubicon