If you are loosing 10-20% with E10 vs E0 there's something wrong with your Tahoe, 02 suburban same engine using E10 and E85 documented miles, my wife doing the driving me recording the MPG she had no concern of MPG or driving habits. Her 15.2 mpg to 12.4 E10 vs E85. I'm not going to mess with the % again, but I know it did better btu/mile with E85 over E10.
2002 Chevrolet Suburban 1500 MPG Reports | Fuelly
30 & 35 fillups. 8800 miles vs 10,900 miles, I'd like to see some documented data, if it drops that bad you need to replace some O2 sensors.
I doubt 98% of drivers would notice 1 or 2% more decrease in MPG E15 vs E10. And some very will may gain MPG. Don't feel like posting the link again for North Dakota/University of MN study on blending ethanol as I have already linked twice in last few days.
Edit: could help it, lost 18% mpg, btu's/gallon down somewhere around 25%.