There are things you can do to improve mileage, and as others have said, slowing down is the biggest one.
Driving 60-65 instead of 75-80 on the highway gained me a 20% savings in fuel economy.
Don't lift the truck, don't put retarded huge tires on it, and don't drive it hard. Those things will save you fuel as well.
I did the math, 10K miles worth, and never saw any measurable mileage increase from any additives. Although I'd suggest a bottle of injector cleaner about every oil change.
But if you do a majority of city driving, you can give up on it now.
That said, I have deleted the emissions stuff, and tuned my truck, and my dealer is still happy to do warranty work. Depending on the work involved, they don't even require me to turn it back to stock about half of the time.
They replaced my upper radiator hose and O rings yesterday, all under warranty, I left my DPF and DOC at home, and they never plugged into the computer, so I kept my Superchips flash on the truck the whole time.
My suggestion is to find a mod friendly dealer.
My truck is now the truck I thought I was buying in 2008.