By the way, another option would be a "pusher trailer"... Not a generator at all. It is just a gas engine, gearbox and driven wheels. When it pushes you then you just let off of the accelerator pedal and the regen charges the batteries while you maintain speed. I doubt anyone is going to "productize" such a thing for you, but you could custom build one. The ones I have seen done tend to maintain registration as a full vehicle so you would need to have it pass smog checks and probably get separate insurance for it... In other words a lot of hassles.
Pusher trailer - Wikipedia, the free encyclopedia
EV Pusher Trailer
EV Pusher | MrSharkey.Com
Pusher trailer - Wikipedia, the free encyclopedia
EV Pusher Trailer
EV Pusher | MrSharkey.Com