I live in AZ and most of the drives that I do have a gas station near by so extra capacity isn't mandatory, but as others have said, they have been there for others on the trail. However my favorite area to go camping and exploring is in the Arzona strip and there and very few gas stations near by. We do lots of roads in four low and really burn lots of gas and would end up having to head into St George, Fredonia, Jacob lake or Marble canyon for gas about every other day or sometimes every day.
So for AZ Strip camping something like the Genright tank would prove a good solution, but very expensive and a fair amount of weight to lug around all the time in my DD jeep. A simple jerry can holder would suffice for most of my trips for just in case and is easily removable when not needed.
Years ago I had a Dastun pick-up and was able to add a 10 gal tank under the bed and accessed thru a hole cut into the bed. I was hoping to see something that is a diy under the jeep made by someone here. A ten gallon tank with a skid plate would just about fill the bill and most likely not cost too much.
I am enjoying seeing all of the ways we have found to carry extra fuel.