The situation:
1995 Grand Caravan sat (nearly) idle for at least a year and a half. Was averaging around 23-24 MPG at the time. ~171,000 miles

It got brought out of retirement in October. At that time, still averaging 23-24 MPG. On a full tank, I could go about 380 miles or so before the gas light would come on.

I got an oil change at the beginning of November. Some time around then, I noticed my gas light coming on much sooner, anywhere between 310 - 330 miles. That's a drop of about 4.5 MPG. It hasn't gone any higher ever since.

I still had my old tires, though replaced them shortly after the original oil change (standard 5W-30). They were the same brand and model as the new tires.

At some point (though much later) I also replaced a leaking transmission cooler line.

I recently got a state safety and emissions inspection and everything appears normal.

A few things to note:
-my cruise control gives up after driving for a while in warmer weather, but seems to work fine in the cold.
-it was October when I started driving it again, so temps were probably in the 40's to 50's
-Since November temps have been very New England winter, ranging from low teens to mid 30's. Sometimes it does take a couple of minutes for the transmission to run at lower (normal) RPMs when I first drive on a really cold cay, though I'm not sure that would be enough to kill 4.5 MPG.
-My EGR valve makes a putting noise while the vehicle is running, though seems to be working. There are no error codes.

I thought another oil change, new tires, fixed transmission fluid leak would all help but it is still much less fuel efficient than it was. My air filter looks okay. The engine runs smooth.

My old 1992 Town and Country got consistant ~21 MPG no matter what time of year it was which is why this is concerning me.

Any tips/advice?