I’m going to guess something wasn’t quit right with the 2005, as you probably shouldn’t have been pulling much over 215-218°. The 190° thermostat used, in both the 2005 and 2020, cracks at 190°, is full open at 207°, and has a max allowable temp of 225°. I would see 215° on my 2005 towing up a 6% grade on a hot summer day at 20K lbs and making around 130/300 over stock power. The only time I ever saw hotter was with a jet ski mounted on my front receiver blocking most my airflow and pulling a 6% grade at 65mph around 20K GCW. That was also before I modified the fan programming. After modifying the fan programming the hottest I saw was 211° regardless of ambient temp, elevation, grade, or weight. I often pulled hills at 90-100% power on my tow tune that made 415/850 to the ground. Despite the differences in GCWR the 05 and 20 have the same thermostat and the 05 had a larger cooling system. The 20 does have slightly better fan programming thou.
The 6.7 holds over a gallon less coolant in the block, so between that and the EGR cooler runs hotter. 213-215° is pretty normal with a 6.7 and any decent load up a decent grade, but they rarely get hotter with the fan operational.
What kind of grades are you pulling with that 20 that needs WOT at 18K GCW? Or are you simply playing with WOT? If so, odds are good the power isn’t sustained long enough to develop higher coolant temps.
FYI, GCVWR isn’t a real term. I think you mean GCW (Gross Combined Weight).