2003 Astro gas mileage

I have just turned 84,000 miles on my 2003 Astro van V6. I have kept up manitinance and oil changes. Just before my last oil change, I checked the oil and was a quart low. Also, in recent weeks, I noticed the gas consumption is horrible. I am running my airconditioner higher than normal. but the mileage is almost twice as bad as last year. The only thing I have ever took the van in for (engine wise), is that the engine light comes on if I accelerate all the way, like getting on to the highway and running over 75mph. The chevy tech showed no informalities when he did diagnostics.