I'm pretty sure he's charging a 12v battery, and using that to run all his accessories so that the HV battery has less work to do.

The cost is high, and the benefit is trivial.
The MPG boost from driving without a roof rack ( less wind resistance ) or reducing 50 pounds of weight, (spare tire+jack?) is greater than the MPG boost from an in-car solar charger.

Solar from a house roof size solar panel is the only way to go IMHO.
You can't get enough watts to make a measureable differece from a car mounted solar system.

Your FEH battery puts out a nominal 5,000 to 15,000 watts, with peak boosts of 25,000 watts. Now take a look at the wattage of your solar cells. 25 watts? 100 watts?
-John

Oh, without a MAJOR hack an MAJOR re-wire and a MAJOR $ investment, there is no easy way to charge the 330v battery.