How many gallons does the fuel tank hold. Is it a square, or an L Tank? Whats the width of the tank? They do make some fairly large vertical tanks, that take up less bed space.
Downeaster makes all different size V Boxes. Commonly you'll see them "advertised" for JD Gators, utility vehicles, etc. I think I had looked at their website awhile back and they had 1/3 yd, 2/3 yd spreaders, etc.
I'm sure someone could custom build one, but it would probably cost alot of money.
Do you have anyway to load/unload the vbox and/or fuel tank. Are you loading fuel in your truck, or into machines on jobs?
The other thing I purchased, but haven't assembled yet is an engine hoist from Northern Tool. Depending on where you need the fuel, you could use the hoist to off load the tank and take it back to where the Vbox is at and load that for the storm.
I don't think I would place the weight of a Vbox on the tailgate. I'm on my second tailgate, from light use of the p/u bed, they just aren't made to hold any weight. And a Vbox would unload towards the rear, and place a decent amount of weight on something thats attached by two cables.