gas milage, heater, oil, gas filter?

My ION's gas milage when it was new was alot better than it is now
at 15K miles. I'm wondering how much if any does running the heater
have to do with this? Also, how much does changing the oil every 5-6K
instead of the recommended 3K have to do with this? Does that gas
filter recall have anything to do with this? I used to put in
11 gallons of gas, and see the low fuel ding around 300 miles. Now
I put in 11 gallons of gas, and see the low fuel ding around 260 miles.
I'm tanking at the same Mobil station as always.

 

Mileage during the winter is lower than during better weather. Wait until
late winter, spring and early summer before concluding that the mileage
actually went down.