Do you buy fuel at same place every time? Just because they went to 10% ethanol means nothing. Here in Texas they can put anything around that number, dealers routinely check ethanol percentages in fuel to verify warranty violations so customer complaining has to pay for the work, of course customer has no idea what's in the fuel. Even though the federal mandated max amount is supposed to be 10%, very often it is more, as high as 20 in some cases. They tilt the fuel with more alky to make your mileage drop so you have to buy more fuel and faster. Occasionally it even gets high enough to cause engine problems, Walmart stations went through that a while back. The finger pointing between Walmart and Murphy Oil was pretty funny.
Try different station for fuel, the problem has been covered by newscasts here in the D/FW area. New cars get good mileage, then suddenly they don't........
If they ever allow 15% as legal watch out. The fuel suppliers claim it will not hurt cars, but they already know since they have been doing it in some cases for years. At first they hated ethanol, but they have learned to embrace it and even love it, just one more hard to trace method for ripping off the public.