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Ed, you have a 2004 so you have EGR but no DPF. You'll see quite a bit of mpg improvement, but not as much as post-2007 folks.
You're right that most of the programmers and other mods are advertised as producing higher TQ & HP, but, in most cases there's significant mpg improvement as well. They're not advertised that way for two reasons: 1) technically, they're not street legal so the companies advertise them "for off-road use only" 2) many guys do use them to increase TQ & HP. Some of that increase comes from burning more fuel, but a lot of it comes from increased efficiency.
It's complicated, but in simple terms high in-cylinder temps boost thermodynamic efficiency, however, they also lead to more nitrogen oxide (NOx) emissions. The programmers make changes to things like injection timing which results in higher efficiency but also higher NOx levels than what the EPA allows.
For post-2007 trucks getting rid of the DPF & EGR and reprogramming can result in over 30% mpg improvement. And I'm not just throwing those numbers around--that's from data I've collected myself.
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