Chris - you are right, forgot about that. It is a great little setup. I have more then enough room for tow straps, cargo straps, washer fluid bottles, 2 hitches, jumper cables, and several clevises and other rigging pieces. My fuel burn was about 360 round trip and I got home with 3/4 of a tank and have yet to fill it yet.
With a short bed you are going to lose space behind the hitch (if you have a 5th wheel.) I have an 8' bed, but even for me when I have to load stuff it just either goes behind the tank or if its long enough on top of the tank and tailgate. You could set it to be removable, mine would mean removing 4 bolts to get the toolbox combo out and come up with a good way to plug the gravity fill line. They also make lower profile ones that stay below the bedrail so you can continue to use a tonneau if you wish. Only downside of a cap is access to fill the tank. If you want to expand the frame mounted tank, look into titan fuel tanks, but try not to get sticker shock. For my 2012 F350 CC long bed its 1500 for the 50 gallon and 1700 for the 65 gallon. A 50 gallon for a 2011-2016 Ford short bed is 1433.00 MSRP.
What I like about my setup was when I sold my 2006 and bought the 2012, I just transferred the tank over. Not to mention the costs savings over the mid ship mounted tanks. If I had a titan, it would mean buying another tank for the new truck.
Getting creative like J did is another option, I used to have a tonneau that fit over the tank, then I went 3 years of it being rolled up so I decided not to buy one for this truck.