First, I would like to go over the basics...
Tires. Did you make sure that the tires are properly inflated? Did you recently change tires to a different size and/or brand? This can have a HUGE impact on fuel mileage.
Engine Oil. Did you recently have the oil changed with a heavier weight than normal? 10W30 is what this engine calls for. Did somebody put 10W40 in it? many quick lubes will do just that. Is the vehicle due for an oil change?
Thermostat and coolant condition. If the thermostat opens too soon, the engine will take longer to reach operating temperature which will cause a loss in fuel economy as the engine runs most efficient at operating temperature. About 195 to 215 def F is normal operating temperature. In addition, poor coolant condition will cause thermostats to open too soon and give incorrect coolant temperature readings to the computer. A faulty coolant temperature sensors will do just that.
Did the weather change? Cold temperatures will cause a decrease in fuel economy just as you have described. Back to the engine operating temperature principle.
Did you change fuel stations. Did the fuel station change fuel formulation? Fuel is a big thing today. Ethanol decreased fuel mileage. As little as 10% ethanol in fuel will decrease fuel mileage by as much as 5%.
Finally... the biggest fuel killer of all. CARBON. The evil little word that consumers still today do not believe exists. As the engine is turned off, there is residual fuel in the intake. This fuel evaporates but leaves behind carbon on the fuel injectors, combustion chambers, and intake valves. The Norstar engines are horrible for carboning up. So, a professional upper intake decarbon cleaning and an injector service would do the trick. NO... NOT the bottle in tank stuff. Actually a professional service. By the time you build up enough carbon to effect fuel economy, it's way too late for a bottle of injector cleaner in the fuel tank. The bottle in tank stuff is meant to keep things clean not clean things.
As for the tune up... ACDelco spark plugs right??? Spark plugs on this engine are to be factory OEM plugs. No substitutes. The oxygen sensors age over time and become slow to respond giving you a decrease in fuel mileage. An O2 sensor needs to go from rich to lean (0.1V to 0.9V) at least 7 times in 10 seconds at 2000 RPM. If not, the O2 is too slow and should be replaced.