First off, you can get a tuner and it wont void your warranty. If you have to take it in just put it back to stock tune and your good to go.
For better gas mileage you always want to check the obvious first like checking your tire pressure, making sure you tires are balanced, clean fuel filter, clean air filter, etc...
Stuff that you can buy that will help are things like sea-foam, lucas fuel injector cleaner, cold air intake, tuner, pulleys, exhaust, and synthetic oil.
I would start with things syhthetic oil. Depending on your trucks miles you dont need sea-foam or lucas cleaner yet. Make sure you get a good filter with the oil change as well.
2002 XLT 4.0, 3"BL, 295/75/16 Nitto Terra Grapplers, K&N FIPK, XCAL 2 w/LaSota Racing Tune, 2 12" Alpine subs, Alpine Deck, Cobra CB, Low Pro Tool Box, Straight Pipe Exhaust, Blacked out heads/corners, Expo OHC/Dome/Cluster, etc...
2011 F-150 XLT, 5.0 Coyote, Hell Bent Steel Leveling Kit, 15% Tint, Flo-Pro Super V Exhaust, MA Audio 1000w amp, 10" Sony Sub