rotopax would be good, a touch expensive perhaps? Looking the most sensible option at the moment though.
Decanting petrol next to a lit gas oven? ...not the best idea I've heard...
I've got the standard Suzuki panniers, I'd rather not cut/drill holes in them, but can tweak the racks a bit if necessary (so long as the panniers still fit). I'd guess ~5litres would be useful, that's about 50 miles for me loaded up.
The best mileage I've got out of a tank was 55mpg, and that was solo, lightly loaded, and being 'careful'... I should be fine, but there's a few long stretches with only a few petrol stations, I don't want to be stuck if one's closed etc.