[JRMichler]

Any IC engine > generator > motor drive system is limited by the efficiency of the engine minus the inefficiency of converting power to electricity minus the inefficiency of converting the electricity back into mechanical power. Diesel electric drive systems are well suited for high power applications such as locomotives. Adding an engine generator and electric motor to your van would add a number of failure points.

Since you list mileage, simplicity, reliability, and repairability as priorities, I suggest rebuilding the engine you have and making the modifications suggested by earlier posters. I also suggest a mileage cam and a five or six speed manual transmission, paying particular attention to the final drive ratio in top gear.

I would put in a multi point fuel injection system. If you want the on the road repairability of a carburetor, keep the carburetor for the throttle. If the fuel injection gets fried on the road, switch the gas line back to the carburetor and keep going.

I would also cover the roof with solar panels and put in an appropriate number of deep cycle batteries. You should be able to eliminate the need to run the engine solely for charging batteries.

I believe that such a system would have better real world mileage than any Harbor Freight generator / motor system.