I think these call for new plugs etc at 100K miles, as per your manual. Sometimes, because that service interval is so long, some components go unchecked until its time for its first tune-up. If it hasn't been done, I'd do it now. The 5.3 engine has a PCV valve at the rear of the left valve cover. You could pull out the PCV valve and hold your finger over it to check for vacuum. Sometimes a new PCV valve will correct a sudden oil consumption problem. Take it off & shake it - if it doesn't rattle, replace it.
In addition, use a strong flashlight to look around for any oil leaks. If you find a leak, I'd recommend discontinuing the Mobil 1 and going to a high mileage oil with seal conditioners. I have seen such oils eliminate leaks and I use them myself in a truck with 203K miles. Of course, gentle tightening of the valve cover bolts to your torque specs helps too.
I also think its possible, as isell indicated, that this may be your new reality. So, you should check your oil frequently. Waiting until oil changes is too long to check your oil. If you are still driving this at a rate of 30-35K miles/year, you should check your oil and other fluids every weekend.