[sanghill]

Roughly 11.8 MPG. Something is wrong.
Since you don't have any CEL codes then the engine is probably fine.
Check your tire wear to make sure it is even. Uneven tire wear means poor wheel alignment which can increase fuel consumption.
Hopefully you are using 87 octane unleaded fuel, or the equivalent. The less alcohol in the fuel will give better fuel economy.
Check the tread depth, make sure it is nearly the same on all tires, all of time.
How long ago did you start getting less good fuel consumption? Did something coincidental happen?
A couple possibilities:
Somebody is stealing your gas.
Mathematically calculate your mileage, don't go by what EVIC says.
Check for dragging brake pads. After driving a while stop and feel the wheels. One wheel hotter than the others suggests a dragging brake pad. The front will be warmer than the rear.
Or, put the Jeep up on a frame lift and turn each wheel to notice a wheel is more dificult to rotate. Or, just jack up one wheel at a time.
Check the exhaust system for visible damage. Make sure exhast pressure can be felt coming out of each exhaust tip. A loose baffle inside the muffler can result in exhaust back pressure.
Find a shop that can do a tail pipe emissions check, note if the engine is running too rich.
Look for fuel leaks under the Jeep. Never, never top up when you buy gas. If you have damaged your evaporative fuel emissions system then all bets are OFF.

Since you have a TH, have you ever topped up the gear oil in the rear gear box? RDM. If not the gears may have increased friction due to lack of oil.

Check the driveshaft joints and see if they are loose or slinging grease. There may be increased drive shaft resistance to turning freely.

Get the battery load tested with a tester that provides a printout. The battery should not be charging at a high rate all the time. Check the battery cable connections.
Make sure the IBS on the battery terminal is working correctly. Possibly the alternator is defective but I doubt it.
A serpenine idler pully bearing or some other accessory bearing may not be turning freely.
If the AC compressor is not working properly it could put an extra load on the engine.

Hopefully you are not driving around in 4 low? Make sure the SSC knob is set to AUTO.

If start/stop is not working correctly then every time you are stopped you are getting zero miles per gallon. You could just turn the engine OFF when you know you are at a long light.

Try a nice long drive on the open highway. Fill up to the first click just before the trip. Fill up to the first click at the end of the trip. Do the math and figure out your gas mileage. Avoid passing everybody. Avoid hilly terrain. Drive gently. You should get better than 20 MPG, about 23 MPG would be good.
This should keep you busy for awhile, good luck and report back, there will be a test.

 

[sanghill]

The front and rear tires MUST have the same diameter or there WILL be drivetrain damage. Refer to your Owner's Manual for the tolerance.
KO2 tires are heavy and probably reduce fuel economy. Apparently you do not go off road. The TH actually goes off road fairly good with stock tires.
The easiest way for you to check your IBS is replace it and see if ESS works. The more expensive way is to have your Jeep dealer check it with WiTECH.
If a sensor was malfuntioning you would get a CEL. If the Check Engine light on EVIC is NOT ON then you don't have any active codes.
85 octane should work just fine and save you money since you are getting lousy gas mileage.
Try to get a vehicle history report. It may give a clue about your fuel economy. In the USA a Jeep dealer can print a VIP report. A CarFax report is also available.
A VIP report only says what happened while the Jeep was within the Warranty period.

This is my recommendation:
Put on stock size Firestone Destination AT2 tires. Rotate them regularly according to the Owner's Manual. TH rotation pattern is different than non TH.
Get a 4 wheel alignment check and check the suspension for excessively worn parts. Helps to keep the new tires wearing evenly. TH suspension geometry is different than non TH. Since you bought the TH used this should be beneficial.
Calculate fuel economy according to the actual distance driven, not what the odometer says. This is a lot easier when you use stock size tires.
Inflate the new tires at ambient temperature to 36 PSI.
Use 85 octane unleaded gas.
I predict a significant improvement in fuel economy and reduced fuel cost.
Download the Owner's Manual as a PDF from jeep.com.