08 fuel mileage

I recently bought an 08 v6 double cab auto. It has 65k miles on it. When i got it i was getting 17.6-17.8 mpg driving to and from work. The truck had 265/75/16 bridgestone revo 2's and the bilstein 5100's. It now has a custom front bumper/winch, icons, and 285/70/17 duratracs. Im getting roughly 15.5 mpg to and from work now. My main question is when im in the hills driving around im getting horrible mileage, like 10 or less. Is this a normal thing for the 2nd gens? I just sold my 1st gen that had an arb/winch SAW's and 33 duratracs. That truck got between 15-16 mpg no matter how i drove it.

The 08 has a trd exhaust and kn airfilter. Im gonna try to clean the MAF tomorrow but other than that anyone have any ideas? I tried to search with not much help, thanks in advance.