Darn, too bad I didn't know because I coulda showed you the 60 gal. w/tool box I have in my dually. It's under my topper, but I switched the sliding window around so I can open it at the front and fill the tank through it. But the window does leak some when it rains.
I've also seen some set-ups where the filler cap is built into the side of the topper. It's made by RDS which are sold by Northern Tools. I got mine at a local truck supply store.
It is gravity feed with a shut-off valve. I believe I read somewhere that you are not suppose to keep it open while on the road per DOT regs. Hope someone can clarify. But I open it at the track and it refills the main tank there.
It's great to be able to skip through the higher fuel price states!