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Although I've never owned one, Trucks with the Toneau (sp?) covers, or cloth tops over the beds are generally expected to get 1-2mpg more. It eliminates that pocket to trap the air.
With the tool box, if you have a short 6' or a standard 8' and the tool chest is 2' you've conscomed 1/3 to 1/4 of the bed space (respectively). Now the air traveling around the cab will have the least effect on the pocket directly behind the cab. This is due to the general poor design of air flow leaving the edge of the cab (rounded corners). So as the air flow comes back down over the top, and around the side windows it catches up near the rear of the bed of the truck. So basically you filled the location of the worst void. With a smaller pocket near the best part of the air flow you may have eliminated/reduced the drag.
Also if you have loaded that tool box with tools you have carrying additional momentum and maybe be seeing increased coasting distances if you are effectively using hypermiling techniques. An additional .1 of a mile over a few coasts on daily drive can add up to a few miles of coasting under zero engine load. And it could be worth a tenth or more MPG on each fill up.
I know you're talking trailer-less MPG, but now with the 5th wheel, are you towing a trailer? or just have the 5th wheel installed because you have a pickup? (people do this, don't get mad, I'm just asking! :-) ) If you've gone from a trailer hatch hauler to a 5th wheel trailer you may be picking up alot of aero from the trailer shape when you dual haul something back there.
Also to play devil's advocate...
you removed the fiberglass cap. Was it one of those very large, taller than the cab, caps? If so you reduced your over forward cross section. This reduces the drag on the vehicle. Considering the truck is probably 5' at the roof, and the cap could have been ~.5' above you could be looking at 2-4 square feet reduction in the cross section. And that would be worth a few tenth MPGs as well.
Did you drive a different route than normal? I can take several different ways to work, but some have a loss of 3mpg due to town traffic.
Did you use a different gas station? Last I heard the government wanted to lace diesel fuel with 2%-5% bio-diesels....if this is true, could you have found a shop selling straight diesel without tampering?
Have you done any routine maintenance lately? Especially something effecting MPG?
Either way how it worked, congrats on the 2mpg! Thats a big win on a 25 gal tank at your mileage. You're basically saving a two gallons of fuel per fill up!
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