https://www.caranddriver.com/features/a36062942/evs-explained-charging-losses/This is a bit more current on actual charger/battery losses.... and they are seeing about 20% losses for charger and battery, which is way better than starting at 35%.
ST head efficiency is more likely around 80% despite the simplicity and high THD. So add another 20% to the 80% for charger and battery losses, and it's still 60% efficiency.
So 6 gallons of diesel for 4 gallons of gas mileage is reasonable. Cold weather is another story; the present LFP batteries need heating in cold temperatures which eats into the charge power/efficiency dramatically. But if the fuel is free that isn't a deal breaker.
The 6/1 is pretty small for car battery charging.
Many Chevy Bolt drivers see 3.7 miles per kilowatt hour. Assuming 3Kwh per hour of 6/1 charging, 12 hours of charging would be 12x3kwh =36 kwhrs x 3.7= 133 miles. If you estimate your full load fuel use for those 12 hours at 0.33 gallons per hour thats 5.2 gallons for those driven 133 miles. 133 mi/5.2 gallons= 25.5 mpg using the CS as generator for an EV.
This is consistent with my previous estimate. The fuel consumption at 0.33 gallons per hour at 3KW is something you can check yourself as this may be off. Plug in your own number and see what you get.