Rapid decrease in gas mileage.

I own a 2003 Bravada with the Vortec 4.2L V6 with about 68,000 miles. It has always done decent on gas out on the highway, averaging 21-23 mpg. This last week was the first time I drove it long distances (more than 30 miles) on the freeway since traveling from Colorado to Michigan in late August. On this last trip, my average mpg did not even reach 17! I started watching the instantaneous reading closely, and it seems to jump around irradically on flat roads at a constant speed. I pulled the air filter, and will likely replace it soon. Still, I don't think that this is the sole perpetraitor of this problem, as the filter is not that bad and this came about all of a sudden. I thought maybe an oxygen sensor might be faulty, but would imagine that to trip the check engine light. Any ideas? Thanks so much!