Here's a few older pics.
This is the 9 gallon tank. I painted it white because a big red tank up there would look silly. Of course a few years after I created this set-up, they started selling those red modular/stackable gas cans that people put on their racks. At least mine's white and I can refuel without taking it down
I made a little custom fitted hinged bracket to secure the tank, it attaches to the non-hinged end with a large wingnut. Very secure.
This is me airing and fueling myself during a trip in New Mexico. I jumbled together some hose, a primer bulb, and a connector to the tank. It takes about a minute to set up and prime it and once it starts flowing it takes about 6-8 minutes to drain 9 gallons. When I need to put gas in the "fuel cell" as I call it, I just stand on the tailgate
This was about 500% cheaper and easier than doing a "real" aux fuel tank, and since my truck is beat up and nothing to look at anyway, it just fits the hard-nosed motif