Full size tractor unit empty fuel efficiency. Best one?

Have a look at Expedition Portal.

There are a variety of different vehicles but I would agree that a modified rigid would probably do the job. Maybe a 6x4 ? Whilst you think it might be relatively light, you would probably have to be carrying your own fluids, freshwater, extended battery systems, and everything else to go with it.

You would probably also want to be carrying some spare wheels in case of puncture in a rural area.