The cable lockers work great, I've used them a few times in Anza Borrego and the truck locked right up and the extra traction was awesome.
As for mileage, I have no idea, I don't really check. It get filled every 5 to 6 weeks.
The tank is a custom model by some company in AU, can't remember the name. Having the extra capacity is a great thing on long trips.
DW