I assume you towed it in 4th (ie. OD off). It appears that Nissans (well the Frontier/Xterra/Pathfinder and the Titan/Amada/QX56) like the revs on the new VQ40 and VK56 so if you see the temps start to rise, drop the gear down a notch and raise the rpm a little. This will get all the fluids pumping more and the temps will decrease quickly.
How heavy was the trailer and any issues with braking?
I don't know what it weighs but I wouldn't expect the Frontier to have any trouble from the look of it as long as the OD is off.
I have never had the temperature gauge in the cluster even move when towing over Vail Pass or Monarch pass but I guarantee the engine temperature went through the roof. I wish there was a factory option to get the actual temperature rather than a gauge that is designed to not scare anyone until too late. Maybe a Scanguage is in my future.