I have a Titan and I am guessing it might be the same case as the Armada. The manual says to tow in fourth gear and not D. Also press the tow mode button.
I get 9-10 towing around here, about 14 mixed driving and up to 19 all highway. My commute to work is under 4 miles so it does a job on my MPG.
The Titan and Armada are showing some age but I feel like they have worked a lot of the kinks out of the existing design from the early years and it has resulted in a pretty reliable vehicle. I am on my second one, the previous was a 2006.