As the others have said. TH mode itself does not remove any gears from use. If you want to keep the truck from shifting into 6th gear, use the buttons on the shift lever and set the numbers to 5. Then, the highest gear it will go in, is 5th, locking out 6th. I've driven my Ram locking out 6th and it towed fine, running at about 1800 RPMs on the freeway doing 63MPH. This past weekend, I left it in D, and tow haul mode, but it wasn't a great comparison, as I was on two lane highways, small towns etc... Next weekend will be freeway travels again, so I will see how it impacts fuel mileage, driveability etc by leaving it in D vice locking out 6th.
Mike