[P-hack]

It doesn't have to be a converted car, and it doesn't have to be a complicated driveline (situation dependent).

If you have a mostly flat trip and constant speed, just rig up a small diesel (yanmar style) and a sprag clutch and chain/sprocket it to a wheel to run it at peak bsfc at the hiway speed. The control is basically throttle, the engine can stop instead of idle, and start when power is requested with minor circuitry (golfcart style). Need an educated guess at optimal engine size.

If you need extra range at other speeds and want to be efficient about it then you need a gearbox/clutch.

The inefficiency of the generator problem does not disappear. Though there may be some warranty concerns with bypassing the plug.