I have a 41gal fuel/tool box combo (67 gal total) from Northern Tool that was $500. But to be DOT legal and not void warranty, the 2nd tank needed a 2nd GM fuel pump, 6-port valve, vapor vent, hoses, etc that cost $700. Replacement tanks were not available when I did my install. You can see pics at: http://www.picturetrail.com/dayle1/
I removed the tool box lid since with a rubber gasket, my folding cover fits flush anyway. With 11.3 mpg towing, I can easily go over 650 miles without refueling. Recently spent 10 days in Canada w/o having to pay Canadian prices.