My gas mileage has gotten pretty bad over the last 6 months, my last tank I averaged 12.5 mpg, I was averaging over 15 a year ago. A month ago I had plugs and wires replaced, it was checked and not throwing any codes. Any ideas?

Timing? Dist. cap and rotor? Injectors leaking?

timing is electric controlled and cant be adjusted, it has no dist. cap or rotor it has indiv. coils for each cylinder

Ethenol cuts MPG

Heat, summer blends. Wouldn't hurt to run a can of Seafom through the tank. I usually run a can a month, about every 5th fillup.

did you add anything like a brush guard?

Pretty obvious I know but have you checked your air filter, tire pressure(and wheel bearings)? That engine was also notorious for blowback and clogging up the throttle bodies. That could also cause your MAF to get all gummed up. I had that year truck(Z71 extended cab) and it averaged 17.5mpg(50% city/50% hwy) till about 150K when I sold it. Your 12.5 sounds low, driving habits being constant. That's my best guestimate.

New Air filter and tire pressure is good. I've been considering the Sea Foam treatment, do you have pretty good luck with it?

Run it in every thing I own. Sold on it. However if the throttle body is really carboned up the only way to clean it is by removing it and using throttle body cleaner. I have taken a chemical spray bottle poured a half of can in it and misted the brake booster vacuum line but be careful not to spray too much to fast or you run the danger of hydrolocking the engine. But running it through the fuel routinely will work wonders. The first time run it down to a half a tank and use the whole 16oz can run it empty, then from there on you ought to be good one can per tank full.

I use Seafoam when I notice the idle a little rougher and a little hesitation. Usually between oil changes. Runs better after treatment. I add it to a low tank before I fill up at the gas pump.

Clean the mass air flow sensor.

Carefully clean the maf sensor with a spray cleaner that don't leave a residue. I always used electronical contact cleaner. Be very gentle as the little wires inside are easy to damage. I've had 2 5.3s and have seen from 15 to 19mpg without a trailer on each depending on the amount of highway and city driving and idle time.
There is a throttle body cleaner I used to run through engines while doing tuneups made by Mopar called throttlebody cleaner. It cleaned out alot of carbon out of the engine as well by spraying it into the throttlebody as the engine idled you would have to manually operate the throttle to keep the engine running. I would get the throttle plates good and clean as well as any vacuum or IAC passages. Yours being a drive by wire would require a buddy to operate the throttle pedal to help it stay running. If you do that you have to change the plugs afterwards. Since you already did a tuneup and those iridium tipped ac delco's that go in there are too expensive to foul out I would remove the throttle body to clean it. If you put a plug other than the iridiums that go in then that may be the issue. Did you change out your plug wires while tuning it up? If you didn't then that may be something else to consider.

one other thing to check.pull the vacuum line off the fuel pressure regulator and see if the regulator is leaking fuel.it will start leaking fuel out the nipple the hose connects to.if bad.