That's likly you're problem, depending how the "flush" vs "replacement" was performed. I do not know if you have the tow package with the 4.7L and/or ATF cooler either.Just had my truck's cooling system flushed and replaced at the dealership. .
May be the 5.7L system is different, I honestly have no idea. But I'll tell ya, I just got back from a 3 week trip out west towing a 7k TT with my Sequoia in the hottest parts of the country and then even towing up the tightest, highest mountain roads in the Southwest of which a 32' trailer can travel. Multiple days of 115-121 degree heat while towing all day long. The hardest day was the Death Valley switch backs entering the West side when temps were nearing the high for day of 121 and then all the way Eastward thru Furnace Creek and Amargosa Valley. Trans and Coolant gauges never budged. Torque app registered a high coolant temp of 212, but was not working for actual ATF temps.