Everyone's driving situation is different. The only real measure is testing one person's personal driving situation, unchanged, from one vehicle to another.
Lots of 5th gen people get 15mpg. I reliably get an astounding 25mpg on the highway. I am not typical, and my case should not be used to imply that 5th gens are inherently fuel efficient. Similarly, I won't use faceless internet people getting 18mpg the way they drive their 6th gen to be representative of them, at all.
Also, to nitpick, the 6th gen Offroad/TH/PRO fuel economy gains happened in combination with a 1" larger tire size. And I think it's likely that people with new 4Runners throwing down 465 lb-ft of torque at 1700rpm are using or perhaps happily abusing that torque.
I'm just giving an example of people I know, and their real-world experiences. If you can manage 25 mpg in the exact same model that I own (minus the KDSS), you could likely manage similar numbers in a 6th gen 4Runner.
Incidentally, 3 of the landcruisers owned by the people I know, are rolling on the stock very mild highway tread 245/70R18 tires. They are the same diameter as the stock tires our 4Runners came with, but 20mm narrower in cross section.
The other two are on optional 265/70R18 tires which are an inch larger in diameter, and the same width as what we had stock, but still very light P-rated construction, and very mild highway tread.
The 6th gen 4Runners are on the stock, light, P-rated tires they came with. Falken Wildpeaks on the TRD ORP, and Toyo AT3 on the Trailhunter.
I could do better MPG wise, but I have larger than stock tires with an A/T tread, and they are load range E, for the much stronger tire casing. Which is important on the 4wd trails my 4Runner is driven on. Being larger, more aggressive, and much heavier than the stock tires, it is understandable that my MPG would drop. But I knew that would be the case before I switched the tires.
I feel pretty good knowing that my old, antiquated 5th gen, is getting comparable MPG to the latest Landcruisers and 4Runners. Which are driving in the same area, but I'm running much heavier and more aggressive tires than they currently are.
Having owned numerous Toyota trucks and SUV's over the years, and even having worked on them at the dealer level at one time, I was excited about these new models. Having driven them, seeing how they are made, and what they are capable of in the real world, I am no longer all that excited about the new models.
My hope is that the new powertrains prove reliable, AND that Toyota improves the material quality and fit and finish. Because there are some REALLY CHEAP aspects of these new models which are just sad, and an insult to long-time Toyota owners.