"How much avgas do the military use in drones and trainers?"
I imagine that you are right about piston engine basic trainers using Avgas (the Air Cadet Grob 109s which share our airfield certainly do) but I would guess that piston powered drones might be on jet fuel / Diesel. Back in the mid 1990s I was slightly involved in a project to develop a 100 cc capacity Diesel powered piston engine because the US military wanted a human portable electrical generator but also wanted no gasoline on the battlefield.