The things that I would do if your truck were in my shop:
1: Test the compression in all cylinder, make sure they're all the same and sufficient (130psi or better).
2: Test the exhaust
back pressure with a gauge, to make sure the exhaust isn't restricted. Should be less than 1psi back pressure, a gauge made for this purpose is the only way to know for sure.
3: Double check the ignition timing, I would want to know that it's dead on. Unfortunately, just because it's been at the dealer doesn't mean a qualified mechanic worked on it. Chevy engines haven't had adjustable timing since 1995, so if you're dealing with a mechanic that doesn't have 15-20 years of experience, they may not know what they're doing here. I'm not badmouthing; I would just check it myself to be sure.
4: Check the timing chain free play. You can do this by putting a wrench on the crankshaft and moving it back and forth, while watching the distributor rotor for movement. If the chain has worn or the gear has come apart, it will skip cam timing, and cause dramatic power and MPG problems.
5: Check the knock sensor
for correct operation. The knock sensor can take the timing advance right out of the engine, and again, cause dramatic power loss and MPG problems.
Those are things I would check right out of the box. I've seen over-sensitive knock sensors that cause this problem, it's worth looking at the scan tool parameter to see if the PCM is picking up knock activity.