I am looking for another fuel tank to add to the stock tank and setup. Something hidden between the frame rails if possible or somewhere under the truck so I still have the full bed available. I would like to be able to lengthen the mileage between fuel stops.

 

I believe I've seen larger aftermarket replacement tanks. But it seems that you wouldn't have enough room to add anything large enough to be worth the trouble.

I installed a 60 gal. bed mounted tank w/tool box in my long bed and lost about 18". It's under the topper and I swapped the sliding window around so I can open it from the inside for fueling access. Gravity feed with an on/off valve as well as a check valve in the filler hose. But you may be able to find some thinner, smaller capacity versions to minimize bed space loss. I don't know of any other options.

 

Tranferflow makes larger stock tanks and midship tanks.