I lock out 6th only under conditions that include rolling hills or steeper. If it is flat, I'll leave 6th as an option.
Whether I am or not, I feel like the strongest tow'er on the road! Last summer we did 6k miles with 4500 pounds and at payload on the truck. It did an outstanding job, with no perceivable difference in power up to 11k-12k feet. We upgraded trailer and now tow 7500lbs loaded. I can feel the weight diference under acceleration and on grades, but it still does it easily.
My mileage with the smaller trailer was 11.5mpg on that long trip. I'm pretty sure that's dropped to 10.5 with the larger trailer, but it's a little early to tell. I just got 10.1mpg on a local 100 mile trip, but that included some long 6% grades and isn't fair for an average.
One tip that I tend to do. I'll take it out of tow/haul if going down very long grades. Otherwise, it will downshift to try and hold speed. Sometimes, that's good, but if the road is straight and clear in front of you, why shave off free speed?
