I installed a 50 gal aux fuel tank/tool box in my 06 Megacab dually from Handy Industries. The filler cap,and all the connections are all under the lid,so if you want you can hook it up to your fuel tank without anyone knowing about it.
I installed an electric fuel pump inside the tool box,and ran it through a water seperator filter that I mounted to the frame,and just T'd a fitting into the return vent line to the filler tube of the tank.
I wired a switch on the dash,and when I wanted to transfer fuel from the aux. tank to the main tank,I just turned the pump on.
*NOTE* You will need to do a test on the pump to see how many gallons per minute it actually pumps,or you run the chance of overfilling the main tank,and fuel will start coming out of the vent cap.
Don't ask me how I know this-LOL!