I realize that it is a bit of a shock to start driving a Jeep and seeing the huge drop in MPG compared to most everything else. As you have seen there are several opinions on this issue. Some swear that a Jeep (TJ like yours) can not get better than 15mpg.
There are a lot of different things that work....but they are machines and will vary from one to another and geographic locale as well as the operator will make a huge difference.
I have an '04 4.0I6 with 3.73 gears and dana 44 soft top when it was "new" it got about 19-20mpg on the highway regularly and sometimes saw 23mpg. Bone stock.
While it would do more than 75mph.....gas mileage fell out the bottom. At 50-55mph it was the 19-20 range. Around town it was 17-19.
I am an old man and drive like one.
Since I have done SL, MML, BL, 5" bigger tires than what it had when I bought it and changed the weight by adding bumpers etc...I get about 15-16 around town...still an old man...... still drive like one...best I can tell you is slow down and keep it in top shape!!! If it is new to you change ALL the fluids and do a tune up!!!