6.7 Cummins runs hot at highway speed and normal at idle

215 really isn't all that hot. What temperature was it running at before in similar weather conditions?

 

How do the radiator/condensor/ charge air cooler look? No debris/dirt packed in the fins or between all the coolers? No folded over fins?

 

Agree^^^^^^
If it only does it on the hwy and not sitting still it's an air flow problem. Your cooling package (rad,condenser,charge air) is blocked or plugged up.
I'd take the front end apart and wash each core thoroughly.

 

Are you sure? Some times the condenser sandwiched between the other cores can be plugged up and you cant see it unless you take it apart

 

Have you burped the cooling system? On a cool engine, open the radiator cap, start the truck, lightly squeeze or burp, the upper radiator hose while the engine is running, if the level drops add coolant until you top it off. Had a similar issue with my 6.7. This fixed it.

 

It’s a long shot, have the cap pressure tested.. A bad cap causing low operating pressure, can cause it to run hot. Or so I’ve been told, on newer systems,