Even if the generator is less efficient than a truck with an ICE engine I think it would be a net win. You would only be using the generator on long off road trips where you're isolated and don't have an alternative way to charge the vehicle. If having the generator means the EV is a viable choice then you would still get the environmental benefits of someone using an EV instead of ICE for 95% of their driving.

Generator->electric->drive gives a longer range per gallon of gas than ICE->drive, since the generator is always in its optimal power band.