On "race" cars the battery is typically near the fuel cell because your trying to get weight over the rear tires for traction. On a wheeler it doesn't really matter where by the shorter the cables are the better. I would not mount a battery right next to a fuel vent though.
Why not put it in the "glovebox" under the seat? or do the trucks not have those.
and, Why won't it fit under the hood?:flipoff:
Put the toolbox on top of the fuel tank, put battery inside toolbox; vent outside.
A plus to this would be having the fuel filler inside the toolbox allowing you to lock it.
If you plumb the vent properly, and away from the battery mount, it's no worse than a battery mounted under the hood, near the fuel system components there that are subject to leaking potential anyhow. I agree with Brad though on the choice of battery..