OP, it sounds to me like the truck did its job and handled the situation. I personally wouldn't worry about it.
But ShinerBock's description of what is going on with lower rpms and EGTs is on point. I would continue to just operate in D. If you see temps rise, or have a long climb w/ your rpms stuck under 2k rpm, downshift a gear and let more air run through the engine. Personally, I downshift before the truck gets to the point of having to do it on its own (only have seen temps rise a couple times in conditions similar to what you've described, so it isn't like it is an every-trip scenario), but the truck is managing the issue appropriately, and I feel confident in saying you didn't hurt anything.
Good luck. These Ecoboosts really tow well. Compared to the naturally aspirated 1/2 tons I've owned or driven, the EB takes the take with a trailer hooked up to it!