Genset efficiency is CRITICAL to something like this being successfull.
My fathers Miles ZX40 kei van (which is electric and weighs 2400lbs) uses a whopping 75amps x 50 volts (on a bad day) to go 25mph
Slow-Turning 5 kW Yanmar Diesel Generator
Using THAT generator he would get 88mpg driving down the road off the genset.
Now compare that to the genset he actually owns
Amazon.com : Honeywell HW2000i 2, 100 Watt 125cc 4-Stroke Gas Powered Portable Inverter Generator (CARB Compliant) : Patio, Lawn & Garden
and you end up with about 25mpg.
If you go with his "old" generator it gets even worse (think low 20's) and this is on a van that with the big 660cc motor gets 25-40mpg stock anyway)
so be carefull, the generator must be very efficient or this simply won't work out very well.