The reservoir was replaced after the first boil over. The dealer said it wasn't holding pressure. They also replaced both thermostats. After they replaced the reservoir, I hooked up the trailer and headed into the foothills. The temp gauge climbed very quickly towards the red 260 - sort of like watching the accelerator climb while accerlating. I stopped, turned around and headed back down the mountain and it cooled down.
I experience no problems hauling on flat land, only while climbing. Again, I have been hauling the camper or the horses all over the mountains with no problems for 3 yrs - this only started after they did their "routine" service. The third time I had it in to them was when they said they are stumped and are contacting the GM techline. This was 3 days ago.