Perhaps a diesel -PHEV hybrid would be the best solution. Local travel could be done electrically. The diesel-- reserved for highway, long-distance travel.

I think most power trains rob a lot of power, so perhaps a diesel engaged closer to the driving wheels would need less HP. And it could take much less power to keep a vehicle moving along at sensible highway speeds, which also would be more in the rpm range where diesels have better power. The electric motor could assist for hills and passing (assuming the battery isn't discharged!!).

The German LOREMO boasts up to 157 mpg on its 20 hp diesel. Ah, but did they test it in the winter? A strong gust of "vind" might send it down the "bergabhang."