throw 350 kg of diesel. So the extra weight is 3 tonnes, not 6. Not peanuts, but I think it is manageable and the math could work, especially if they charge on off peak or surplus electricity.

A diesel electric hybrid, at least naively, makes quite a lot of sense to me. Your generator can be sized such that at full output at its torque peak it's making enough power to push the truck at highway speed up a slight incline (just slightly overpowered for maintaining highway cruise). Batteries take up the slack for starting, pulling hills, etc. Remains to be seen whether it actually works.