Weemcgee

I have a 2009 Silverado with the 5.3 and fuel management, (90,000 km/55,000 miles) over the past 4 months I have had some issues with increased fuel and oil consumption. I have registered the truck with GM so that they can monitor the amount of oil that is the truck is going through. To date they have replaced a few oil lines and given it a once over but the issue is still there. The part that kills me is that I was getting 520 - 565 km (320 - 335m) in the city and now I am lucky to get 420 - 465 km to a tank . I have had the truck in about a dozen times but nobody seems to be able to tell me what the issue is. I have had them look into the fuel management system but they say it's operating without issue? I have had them check the throttle control and again no issue? I have even had them look at the drive train to ensure it wasn't running in 4 wheel drive? I am at a complete loss on this and am hoping somebody out there can help..
Thanks,