Gary,
I have been towing heavy with my 2006 (21500 lbs) and I have seen the engine temps start to spike, But most of the time if I keep pyro temps in check there is no problem. You can't rely on the speedometer but must watch the pyro. It is surprising how little you can back off the throttle or as Robyn refers to it as the loud peddle, and the temps will drop. Dropping down a gear doesn't always help but got to love the Allison.
I know my radiator stack is clean but have you checked yours? Remove radiator and clean ac condenser and oil cooler from the back side and clean out the radiator while out. Might want to check your air filter too.
On the highway if you can tuck in behind a semi it helps too. At least with my fifth wheel I have a very large sail that catches a lot of wind.
The main thing is to learn what your truck is capable of and work within those limits. You can work it hard, just don't over work it.
"The Constitution is not an instrument for the government to restrain the people, it is an instrument for the people to restrain the government."
-Patrick Henry
A5150nut
2006 K3500 D/A
94 6.5 4x4 5spd Sold