Agreed with regard to the project dying if it can't charge the main battery.

Supposing that's not a problem, if the GP is right about the Ramcharger using an ICE to run the electric motors, then in principle we pretty much already know it's possible from a power density perspective.

I suppose one could start running the generator at the beginning of the trip instead of waiting for the main battery to drain significantly. Perhaps you couldn't get "infinite" range, but you could stretch your main battery's range out far enough that for all practical purposes you wouldn't care—you could either get to a charger or find some diesel and wait an hour or two after a 10+ hour trip for the generator to recharge the main battery.