That's a nice rig, but mileage-wise it's not so easy to upgrade it without mods. Regarding the engine, you could easily get the cam regrinded for a longer intake valve timing (just like any "Atkinson" engine used in the hybrids) and a supercharger to generate backpressure and overcome the power and torque losses from the discompression due to the longer valve timing...
Anyway, shaving it a little to decrease the drag (removing those roof bars, getting a windscreen-mounted antenna for the radio, among other tricks) might already improve it a little. And in such a big, heavy vehicle I bet you can find lots of spots to take some dead weight away
