It isn't that. He should pull 5th easily and pull 6th even up mild inclines. What he described is not normal.My 2019 6spd 3.6 gets great fuel mileage in 5th at up to 70mph. If I go to 6th, it is really for 75mph and over or you are out of the power band and the 3.6 doesn't have the torque to move the heavy vehicle. The auto is much, much better on the highway. The computer does it all for the driver. Lots of shifting and slipping in the auto to maintain 70 on various roads. I love my manual, but hate it on the highway as the gearing is too tall for 6th gear. But, I get 24+ in 5th at 70. I have an Altitude with 32" tires. The good thing, is that the 6th gear will last a long time They had to have gears to make the jeep go comfortably at 85mph, so 6th it is.
Mine pulls mild inclines in 6th just fine with 35s. It should at least do that we'll.
OP I still would not rule out a slipping clutch. Get @JeepCares involved. Make the dealer do the clutch test with you watching, or do it yourself. If I lived near you we'd do it together.
What tires are you running?