If the coolant isn't escaping there shouldn't be any issues.
How much does the guage rise? What do you define as big hill?
Diesels don't generate much heat on the flat, not really working too hard.
Once you hit a hill and the turbo starts cranking up and fuel is getting cooked, then you will get some movement in the gauge.
One thing that may be worth checking is the water pump. The impeller and inner housing can erode, reducing efficiency.
Don't let the b'strds get you down!!