Peak torque on the 3.5 Eco in 2013 is 420 @ 2500 rpms.... So, if your engine needs 4250 rpms to get to it's 380 number, then it needs to downshift to get there..
Why you didn't get as many downshifts with that 'heavier and less aero' trailer, no one is going to be able to tell you that without being there..
All the previous posts is saying is that since we weren't there, they are only telling you what works for them when they fell their trans is 'hunting' for gears...
How that applies to YOU is up to YOU to decide..
I kinda figured your dealer would tell you what they did pages before I read it.. ;)
Anyway, best of luck and just enjoy the tow.... These new trucks are great and even the 5.slow is a decent engine when allowed to get into it's power band..
BTW, love my Eco and 50,000 miles so far and still as strong as ever.. :) :)
Mitch