Wow you guys have been so helpful, thank you very much indeed.
I will re/read all your responses and work from there, plenty to absorb and think about.
Paul, prior to this happening I have found the sweet spot for towing with my rig and usually drive it that way.
It usually sits on about 2600 RPM, not in cruise control and a selected gear that gives good torque up and down dale, so to speak, and it stays relatively cool, well below half temp.
I have to admit on this occasion i was on the Hume, in cruise control and going a little faster than probably should have been which was fine on the long open flat stuff, until the big hill happened.
So from now on I will drive it a bit more sparingly and keep the speed down a bit and travel like i usually do anyway.
Thanks again for all your help.
Leon