My dad picked up a combo tool/fuel box from
http://www.thefuelbox.com/. His the FTC60. He got the optional plumb it in to your filler neck option. Works great. As the level gets a little below half tank on the truck, the auto-controller fills the truck tank back to about 3/4 full, and continues the cycle until the aux tank is empty. The aux tank level is displayed on the little control box. It requires tapping in to the truck's fuel tank sending unit (easily found in the wire loom running along the frame rail).
The spray on bed liner type coating is nice as well. There are other cheaper cost aux tanks out there. This one is an expensive purchase, but money well spent.