A gas powered pusher trailer would make much better use of the gas, the generator->charger setup would be very inefficient.
A guy named sharkey did make a gas pusher to push his EV long distances, used an automatic trans to keep it simple to control from the cockpit (still would beat a generator->charger by a wide margin probably), basically parallel through the road and if his car has regen he can do some emergency charging with it while driving.
