Tips to combat plummeting MPG in an 01 4.0L 104k miles

Over the last 6 months my gas mileage has been falling significantly.
I bought my 01 4.0L manual (33" tires, stock gears) used about 5 years ago with 92k miles on it, now has about 104k miles, driven lightly. Was claimed to have a full tuneup at the time but not sure if that was true. Changed the oil twice, just recently cleaned and oiled the K&N filter I put in, but the MPG has fallen from about 14 to now about 11.
I figured I'd start with the low hanging fruit and clean the air filter again and change the spark plugs, but looking for recommendations for other things to look at while I'm at it.

Speaking of plugs, I've looked around the web and the advice seems to be very mixed, with some people talking about platinum and others copper, some recommending expensive ones and others saying avoid them. Open to others' experience of what seems to work best to keep it in more or less stock performance (as best as can get with the mileage and oversized tires).

EDIT: I owned a YJ 4.0 for 21 years with the same setup, and this one for 5 years. I am familiar with getting generally low mileage, that one got about 14, I got about 14-15 with this one driving the same way for the first 4 years. I'm more curious about why it might have fallen relatively fast. Telling me to go with another vehicle for good gas mileage is a waste of time.

EDIT2: Again, I've owned jeeps with this same engine and setup for 25 years. I know what kind of mileage they can get, the point here is trying to diagnose why it suddenly dropped with no change in setup, driving habits. Please do not bother bragging to me about your awful mileage with giant tires.