To expand on maintaining speed on long downhill :
1. Have your a/c on - it adds some drag.
2. Ideally, be in a low enough gear where speed doesn't increase, or at least requires VERY little application of service brakes to reduce speed. If frequent use of service brakes is required, downshift another gear.
3. If possible maintain speed the 18-wheelers are going - the downhill is not going to last forever.
Dealing with a long uphill and rising water temperature :
1. Have your a/c off, worst case heater on to help with radiator cooling.
2. Use lower gear to increase engine rpm to increase flow through radiator.
3. Again, reduce speed to that of 18-wheelers, but gear down to improve cooling with more rpm.