Where I live here in the mid south they have what the locals call "attic flies". Not sure they are an actual type of fly, but the ones I have once a year are big, black, and slow. Easy to swat or vacumn off the windows. Your TT ceiling will have a gap between the ceiling and actual roof where the AC duct and wiring run. The flies you hear are likely either in the ducts as you suspected or in the open space between the ceiling and roof. For the AC ducts I would turn the AC to the fan position which will pull air in from the cabin and exhaust it thru the vents. After starting the fan, spray a inside type bug spray into the intake for 4 or 5 seconds. Then immediately leave the TT and close the door. To deal with the deadspace above the ceiling, you can remove ceiling speakers and other openings and spray the bug stuff in thru the holes. Then close up the openings. As a final treatment I would shut everything off and set off a bug bomb inside. Close everything up and leave the TT for 2 or 3 hours.
My last occurance in my house was in a bathroom and the flies apparently were in the exhaust duct fan in the ceiling lite. I sprayed the bug spray into the vent with the fan running. Had to repeat it 2 or 3 times over a few days. No problem since. Around here they seem to be a fact of life and I fully expect to see them again.