I had the same concerns the other week. I'm towing about 7500# Jay Flight 28BHBE w/ a 2012 burb 5.3 w/towing package. Went to Disney down I-95 doing about 60-63 MPH. Outside temps were a steady 102 and trans temp leveled out at 216 for our 8 hr drive. engine temps rose to about 220ish, but was steady. I did have my trans fluid changed (not flushed) and a new filter just for the trip. Trans temp was one of my biggest concerns...getting stuck in traffic in hot weather, just creeping along. But we survived.