[oldtamiyaphile]

As long as you have an auto trans the engine will have to be idling while in motion. Pusher trailer, genset, whatever, isn't going to help much, you're going to be idling a 318 just to lube the trans, turn over the power steering and provide brake vacuum. I can hardly think of a less efficient set up.

Start with aero, E-Fans and an electric power steering set up. You can then add a manual conversion and finally you'll be ready for some of your ideas.

If it were me, I'd buy a Prius (etc) and build a ~800lbs teardrop camper trailer (you'd still have an enclosed space with proper mattress and a usable kitchen). Modern cars tend to need less servicing, not more.

If your van weighs 4 ton I don't think you'll ever meet your target. At around 2.3 tons with a 1.6 EFI engine and 5 speed, my 2014 EMMA+Trailer only manages low 20's (this combo cruises very close to BSFC).

Even my 2 ton six speed TDi van only managed high 20's, much better aero at only 0.33 vs the .40+ of the Dodge.