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.
> 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
They have the advantage building the powerplant (and all its periphery) into the chassis. That saves a lot of material on engine mounts and covers, you can skip the cabin heater heat pump, ect.
A big difference is also if you design it for towing or not. You can probably get functionally infinite highway range with a 20kW ICE if you don't want to tow. And those you can build real tiny. You can probably do with a 125cc with the cutest little turbo the world has every seen.