I do not tow but I do have a lot of mountains in my area. I switched out my thermostat to the Stant Superstat and from the research @DNAdan did we saw it flows more and from my monitoring it keeps temps a lot more consistent.
When you are driving are you locking out 6th gear? or you are you letting boost do the work? I would assume locking out 6th gear would help drive the waterpump and keep rpms up so you are using less boost to try to help with overheating.
Also, did you happen to use tow/haul mode?