[oil pan 4]

I figure intentionally under sizing, or sizing it to your average power consumption would be best.
That way it can run flat out staying near its best fuel consumption per KWH any time you are going faster than say 55 to 60mph.
Then when you pull into town, stop or otherwise slow down the gen set would be putting out less power in theory, but the natural under size of the gen set would mean that its not too far out side its best FE/watt production even when you are sitting at a stop light.

Best of all you can turn off the gen set.
Problem I see is a lot of FWD vehicles do not recommend flat towing, which is what it would amount to if you tried to shut off your FWD drive train based push trailer and keep driving under electric power.

What I have observed over the years with diesel generators is if you compare running them in stand by or nearly no load to running them full load fuel consumption doubles.
Then if you run them at half load fuel consumption typically goes up by about 1/3 over running idle at speed or under very light load.