2012 Kenworth T800 High Coolant Temps Under Load - No Leaks

Towing heavy through the Rockies and my temps are climbing to 225-230 degrees. I have to back off the throttle to keep it from hitting the red. As soon as I crest the hill, it drops back to 190 quickly. Water pump was replaced last year. I don't see any bubbles in the surge tank so I don't think it's a head gasket issue. Fan clutch seems to be engaging because I can hear the roar, but it's not staying cool enough.