Gradual 20% drop in fuel milage

It's been 7 years and 108,000K since I bought my truck new, and for those 7 years i've been measuring my daily fuel consumption on the DIC and it's remained virtually unchanged on my daily commute to work. Between about October to now my fuel consumption has been increasing by over 20% more during that same drive to work.  I could usually make it to work in 2.8L of fuel or less depending on traffic and weather conditions but very rarely is it ever more than that.  Now my daily commute can suck up to 3.4-3.8L or more per trip.  And over those months i've probably blown though over 150 liters of more than I normally would during that same period a year earlier.  I always fill up at Shell, have a clean air filter, run synthetic, and inspect and clean my MAF yearly.

In December I noticed it was a major pain in the ass to fill my tank, it's an issue i've had taken care of years ago by replacing it but it had returned and has since gone away in the warmer weather again.  I also noticed what sounds like an occasional whistley sound when cold (similar to the sound of a throttle body spacer).  I'm unsure what to check next? Evap system, PCV/EGR, exhaust problem, some other weird valve or vaccum issue?  My engine hasn't thrown a code and the only modifications i've done to my truck are an Airraid MIT which has been installed for years and still looks firmly attached.  But it's a mystery to me as i've run my truck in this identical configuration for years with consistency until very recently.  Your help greatly appreciated so I can put that money back into my pocket and not at the pump!

Try checking for an air leak between the MAF and Throttle body.

Since your distance driven is getting up there change the plug and wires.

Good call on the plugs and wires, it was in the back of my mind but I think I'll do that once the weather gets nicer in a couple weeks!  I did double check all my connections from my airbox to the throttle body and all looks good there.

You don't have excessive miles on the plugs, but they are 7 years old.

PS don't get fancy with the plug good ole AC Delco are fine.