Lost fuel avg

It sounds like you are speaking of your Davco Filter. It sounds like whoever replaced your filter pinched on O ring and you are getting air into your fuel system, the BUBBLEs you spoke of. Get a new Davco Filter, you need the O rings, not the filter, and change the O rings.

And yes on winter fuel, that is always good for a 1-2 mpg loss in fuel mileage. Can be even worse if you fuel at the Flying J outfits that add bio to the fuel. I would avoid that stuff at all cost, I hate water in my diesel.

 

I agree with everybody else stating you are getting air in your fuel most likely from your fuel filter could be just as simple as the filter not being tight enough as was the case on my truck a couple of months ago had the same problem loss of power could see air bubbles in my fuel water separator finally I went and tightened the filter and it was all good after that good luck

 

It's winter, colder air has higher density is decreasing aerodynamics more drag/resistance against the truck cross section, more energy is needed, it;s like pushing through water ,slow down if you want a better mpg.
Even the sound travels slower through dense colder air, interesting graph . Density of air - Wikipedia

 

Oil. Was it changed with the same brand and grade? Thicker oil will cause a bit more resistance in the engine.

Fuel filters. Correct filter installed? Bad batch of fuel plugging the new filter? Fuel filter installed correctly?

 

Ok, let' back up a bit. First, what truck and what engine? Second, do you currently have any warnings on the dash or just a symptom low power? Does it come and go or is it constant, only under load? If you have a runability issue get it fixed and your fuel mileage complaint will likely return to normal. As others said, don't trust the dash and do the numbers on paper to compare to the readout to determine if it's accurate or even close.