> 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.