I really doubt that constantly pulling around a trailer would be justified by a slightly more efficient engine on the highway. Mr Sharkey himself says that pulling the trailer nearly doubled the power requirements of the vehicle. I think he was exaggerating a bit but you get the idea. Both diesels and electric drive trains weigh a lot. His set-up was a compromise so that he could get his EV to cross country events. If you were going for a pusher i would go for an electric fifth wheel or a small PM electric motor mounted like a supercharger to reduce the engine load. We're discussing doing one in our EV forum
here.
If you were using your electric around town then why would you need the gasser at all?