Yikes. I’d schedule a long (200 miles or more) road trip, carefully monitor gas use and miles travelled, see what I got.
All i can say (and you no doubt will disagree with me…) is that if the engine is getting too much gas the CEL will illuminate and there will be diagnostic codes set for rich running.
One easy to get to spark plug could be removed. If the engine is really using that much fuel the tip will be blackened and quite likely so will the exhaust tip at the back of the vehicle. The exhaust should also have an acrid smell.
If the tip is clean there is no problem.
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given that, if I were diagnosing this, I’s want to ride as a passenger for a week or so to check driving habits.
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Not sure why I’d disagree with you, what you’re suggesting is perfectly reasonable. The thing is, they’ve ostensibly checked all of those things at Jeep (selling dealer, servicing dealer). If there were an obvious problem, they would have tried to address it. I don’t doubt the sincerity of the servicing dealer at all, I think they’ve tried to find the problem and simply can’t. @BillRussell, the car’s got decent headroom, but probably not for 6’10". 
I don’t accelerate quickly or brake quickly. I never rev the engine. The engine rarely goes above 2500 RPM, the lone exception when accelerating to get onto a highway with a short ramp. I’ve never had a car so severely underperform fuel economy estimates before. And though where I live is technically a city, it’s really more of a suburb, and I rarely drive in heavy traffic.
The few reports of 15/16/17 MPG are the only ones I’ve seen, no one complained of fuel economy that bad on the Jeep boards. Perhaps it’s a problem that plagues certain models, maybe some configuration of parts results in a system error that causes fuel to waste. No idea.
All original equipment as listed on the factory sticker - wheels, tires, all? Does it reach normal temperature pretty quickly?
It may help to contact the carmaker as described in your manuals.
Good point about temperature, it might have a defective thermostat.
Yes, everything does seem normal. I never get any alerts of consequence, except when it gets crazy-cold and the tire pressure goes down (which I address promptly). And I have made no changes whatsoever to the car (factory everything).
Does the heater warm up quickly?
Nearly every thermostat that I replace is prompted by a check engine light. If the thermostat in your engine were to fail you would likely see a check engine light on.
Just grasping here, but what about the possibility of a flaky engine coolant temperature sensor? Those are easy enough to test although one would think a faulty sensor would set a code.
Grasping here, but so is everyone… 
Tire pressures at spec?
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along with this: tire sizes at spec, too? Have you added bigger tires? Off road looking beefy tires?
I wondered the same thing, but he says no.
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That was my thought also.
Right after my brother bought a new Hyundai Azera, he noticed that the gas mileage was really poor, and he also noticed that there seemed to be a fairly strong gasoline odor in his garage. Sure enough, the dealership found that the engine’s coolant temp sensor was bad, and the engine was constantly running on a very-enriched air/fuel mixture. After that sensor was replaced under warranty, the car has been rock-solid for the ensuing 4 years, and his gas mileage increased dramatically.
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There could be one sensor - not working right - for the engine control computer (ECM) and another for the gauge.
Has anyone checked for low coolant, causing a sensor not to be in the flow?
First thing I’d try is take the Jeep on the freeway for a 100 mile trip at 60 mph constant speed as possible all the way. Fill up beforehand, then refill at the end. Using the distance and amount of gas used, compute the mileage for 60 mph. If that’s way off from what you think it should be, at least you’ll have a controlled experiment to take back to the dealership for them to explain. Otherwise the dealership is just going to tell you it is your driving habits (which it could be, even if the same habits don’t affect your other Jeep as much. How driving habits affect mpg varies from car to car.) & don’t depend the mpg dash display if you have one, those are notoriously inaccurate. Also you odometer could be off, so check that.
If you discover you definitely got a mpg problem, the first place to look is the engine operating temperature. Make 100% sure the engine is operating at the temperature it was designed to operate when fully warmed up. After that, use a scan tool to measure the fuel trims.
There could be a half gallon difference during each refill, that is a lot for a 3 gallon consumption test. Driving 100 miles without a destination seems like a waste of fuel.
Shouldn’t that be corrected while the vehicle is under warranty? Would you tell a customer that they are all inaccurate or would you check for software updates and see that the pinion factor was inputted correctly?
Never once got a check engine light. No indication that there’s any issue with the thermostat. Heat worked fine all winter. The car is exactly as delivered by the factory, no changes (even by dealer). Every diagnostic Jeep has done on the car (codes, etc.) has come up normal. And yet, based upon any reasonable expectations, I’m literally throwing away one out of every three gallons of gas that I’m putting into the car. I really appreciate everyone taking the time to try to come up with possible solutions to this problem.