RE: Long lasting gas powered Sailplane
In theory, one could use an electric powered plane with a generator, and fly infinately. This is something I will test on my predator I'm building. One plane(the first one) will have a 50cc gaser(better to have enough at lower rpms than not) then I will have another plane out of the mold right away to electrify. There are companies making generators for electric powered heli's, and I will addapt one to mine. It will power the motor batteries, as well as the FPV gear, and lighting system... Good luck on your search for the perfect platform.
P.S. My plane is on the scratch building forum under-predator b uav build 1/5 scale if enterested. I want to Fpv as well as autonomous one day so this is my perfect solution for me.