Hey my recently acquired Disco1 has two scuba tanks where the main petrol tank used to be. I know you said you wanted to keep petrol but I've still got a sub which probably holds around about 30 litres or so and it actually goes a bit of a way on that alone if I let it, plus I get lots of gas and ALL my cargo space. Of course I need all the space because I lug band gear around a lot and if I intended to take her on a long trip into the middle of a dessert or something I'd probably want a bit more range out of my fuel.

Still, I really like my setup, it still gives me plenty of range.