MPG loss and what I discovered
 

'11 Tahoe 5.3 125k miles.
MPG has been dropping slightly so I figured it's time to clean the MAF. Paper filters only. After cleaning MPG dropped. WTF, mileage is supposed to improve with cleaning.
Replace MAF with GM part, no change, highway mileage in the 16's down from 19's. No codes in any of the 4 code families.
My Autoenginuity PC based scan tool has a MAP testing function, tests 1 and 2. Idling on real time mode test 1 would pass, then fail, then go back to pass again. Test 2 stayed in pass on a 5 min. run cycle. Scan tool will monitor both at the same time. No P, U, B or C codes set. Engine off MAP would read 14, right in range as I'm at 750 ft. altitude.
While waiting for a GM MAP sensor to be delivered I noticed when monitoring the dashboard real time MPG monitor that decel. mileage on a long coast would jump from the 90's to the 50's back and forth all over the place without touching the gas.
New MAP in and mileage back into the high 18's on first 250 mile run, real time mpg acting like it should again.
So my MAP had issues even though no hard code was set. I would never suspected it if my scan tool didn't have a test function for it.
Just passing this along incase others are scratching their heads like I was.