Up here in the north where logging is the main industry, bed mounted fuel tanks are very common as everybody that works heavy equipment out in the bush requires a truck to support it. Many have 150 gal "tidy tanks" with a 12volt pump, to haul diesel, however in recent years combination fuel tank/tool boxes made from aluminium have become very popular.  I've seen a few web sites for the tanks, and combo tank/tool boxes.  Unfortunately, these get to be so large that I would have to consider it a permanent part of the truck.  Whatever goes in, must be able to be removed easily, otherwise, it defeats the purpose of having an 8 ft bed.       I also buy marked gas for the plane, which is simply fuel with a purple dye to indicate that no road tax has been paid. I'm uncertain and curious as to whether you have a similar deal in the US?  One of the mobile MRI truck drivers told me about off-road fuel that was sold for the farmers.  I do recall that he said it was a different color so the DOT (department of transportation) cops could tell if you were illegally using it in your truck.   Naturally, he was speaking of diesel, but perhaps you can get off-road gasoline as well.  I'll ask around about this.
 
Thanks,
Rusty