3.92 owner here, my first questions would be what part of I-95 do you commute on, and where will you be towing the future travel trailer? If youre down south like me and the roads are pretty flat, the 3.21 gears wont struggle as much when towing so you might be okay with those, and pick up a few MPG's. But if youre in the north or plan to tow up any mountains id say go for the 3.92 gears, the fuel economy gains from 3.21's diminish in the north.
It's not so much about the 3.92's being able to tow more, you'll run out of payload
way before towing even with 3.21 gears on a lmited trim truck as others have said, its about the fact that the 3.92 gears tow easier, I tow cars more than anything so not an apples to apples comparison but I never have any issue with power and even towing 8900 pounds the truck didnt strain with high rpm's, itll even go into the overdrive gears at highway speeds, the 3.21 guys with campers seem to complain of really high rpm's when towing so the journey to your campsite might not be as enjoyable. Speaking of RPM's I turn about 2000 rpm at 70 and my best highway mpg was 19.8, I average 17.5 though because I like the sound it makes when i put my foot to the floor.
Just some quick math here regarding fuel savings, my truck gets 17.5 and ive got 40,000 miles on it. Over those miles the avg price per gallon was $3, so thats ~$6850. With 3.21 gears you realisticly get about 1-1.5mpg better, that would cost ~$6300, so about $550 over 40,000 miles. Wether thats a lot of savings or a little is up to you, but if it were me i'd go for 3.92's