I have a 2007 Yukon 4WD I obtained six months ago. I now have 12K miles and in the last month I have noticed my gas mile plummit. I had been getting 19 miles to the gallon and it has dropped to less than 16. I drive 100 miles daily, all highway miles. My dealer is clueless. Anyone have a suggestion? The service engine light did come on once and went off by itself. Delaer says no codes were stored.