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MPG,not as good as it was

I have an '02 GMC,Sierra,4.8 L 5-speed,once upon a time this truck would easily travel over 500 miles on a full tank of fuel,for the first 10 years ,on a road trip ( which I do every summer),approx.,210 miles one way,this truck would still have just over 1/2 tank of fuel,as of the summer of 2012,the thing used up to 3/4’s of a tank,Then the service engine light came on ,I replaced all sensors,O2’s new injectors ,motorvac, new plugs ,wires etc,( All done by a certified ASE mechanic),after his work the truck got back up to the afore mentioned “good mileage”, and the Service engine light went out…for awhile,now it is on again,as mentioned earlier,the truck has basically had a complete tune-up,so whats up with this ??,I sure hope I don’t need to R/R the engine itself,thanks for any/all help, Mike

Check the egr valve?

@orangetruck for starters, please post the fault codes?

There are hundreds of possible fault codes
There is no point in guessing what is going on
This needs to be properly diagnosed and repaired
Once we know what the fault codes are, we’ll help you as much as possible.

As @db4690 says, we need to look at the CEL codes to be able to say much.

It could be a lot of things. I guess if this were my truck I’d bring all the routine maintenance suggested in the owner’s manual up to date, check and adjust if necessary the valve clearances, and do a compression check as the first order of business.