[NachtRitter]

Probably a crazy thought... but I wonder if you had an arrangement similar to Sharkey's (EV with diesel pusher for long trips), if it would be possible to use it similar to some hybrids ... use the EV only for portions of the trip and then when the battery gets low (or when the speeds are higher), fire up the pusher to not only push, but also (potentially with a heavier duty alternator) provide a charge to the EV (via umbilical). Obviously that will add load to the pusher engine thereby reducing the pusher FE, but there'd definitely be excess capacity on the pusher engine (especially when at cruising speed). Plus the combo of EV and pusher use should still give some pretty magical FE numbers, as they do with the Volt and similar hybrid vehicles.

EDIT: I'm assuming here that recharging the EV by engaging regen and having the pusher "overcome" the resistance to forward motion would be much less efficient than recharging the EV directly from the pusher alternator... though admittedly I don't know if any alternator could provide enough juice to recharge an EV's pack in any reasonable amount of time...