Where is the Air Filter located on P3RA-036K?

Question

This is an iutside unit. I have looked inside the Mobile Home and can not even locate anything but the Floor Vents and the Air Return Vent. I have a Filter in there but there shoiuld be one on the Unit, right? I bought this place used and there are no Owners Manuals. The Lady I bought it from was the 2nd owner and she bought it in 2005 and said she never changed anything except the Floor Air Return Filter. My Electric Bill is way high so I think there may be a problem. Anyone out there have any idea where this Filter may be locaterd? I looked all around the unit and all Panels seem to be permanent.
Thank you