Cooling issue on highway

That 6.4 comes with 203 degree thermostat stock. Means the factory doesn’t even want that engine colder than 203. If the 6.4 is stock, there’s no reason to run it colder than what the factory designed it for.

Honestly I’d be inclined to change the fan programming so it doesn’t come on until 200 and see what it maintains without the fan. Make it on at 200 and off at 195. Or on at 205 and off at 200. Still colder than the factory thermostat.