You wanna talk about shitty gas mileage? Try the 1997 F-350 one ton dump truck I used to have! It was a 5 speed with a 460ci gas engine in it. Loaded or unloaded, it didn't matter. It always got less than 10 mpg.
Back when gas was under 2 bucks/gallon I was going through 75-100 bucks/day when the truck was in service all day long. Even still it was a great truck, and I wish I had it now. It might have been a gas hog but it was pretty reliable. Never had a problem with it, and most of the time it was hauling 4-5 tons of blacktop and a lowboy trailer with a roller, sometimes a bobcat and various tools. Not to mention the spreader box that weighed a good 250lbs that was winched up to the tailgate. All that..and it wasn't even a superduty. It was a regular ol' F-350 XL dually with a dump body on it. It was a kickass truck. I have some pics of it somewhere..I'll post them on here sometime. Man I miss that truck..but I don't miss paving.
But as far as fuel mileage is concerned, I don't see what the big deal is with the Viper trucks. Most owners don't drive them everyday and rack up a lot of miles on them, so they're not guzzling nearly as much gas as they are in the same given time period in their daily driver. Plus I think we all agree it's been proven these trucks can get above 20mpg on the highway. Not too bad for 505 cubic inch engine. These trucks gulp gas at WOT, but they should. They're actually pretty average as far as trucks go when they're not being driven hard.