'12 5.3 4x4 mileage

I very new here but I have seen some ppl talking about mileage and I know it's different between driving habits, trucks, elevation and along list of others. I bought my 2012 sierra about 8 months ago and back then I was getting 17-18 hwy about 5 months ago it seems to drop over a month or two and I cant get back into the 15s. I'm usually 14.3-14.7 since about 3-4 month ago. I got new tires about 5 weeks ago and got some Toyota rt 33x12.50s I had 275/70s before so I know going with the bigger tire isn't helping my mileage but I haven't seen much of a change -.1 or -.3 with the new ones. So I have tried looking up to see if anyone had any issues like mine. I know winterized fuel is supposed to be the cause of some winter mpg. Diesel I get winterizing but I have no idea why gas has to be treated for cold weather. So if anyone has any ideas or seen drops in mpg like that? Never have seen the check engine light on this truck and haven't done any crazy repairs or mods either. I just started tuning this truck last weekend so that cant be it. Any input would be awesome. Maybe It would help if i posted a log? Thanks for your time guys!
Sierra 1500 5.3 6l80
Cat back flowmaster y pipe, 33x12.50s, leveling kit