I have a 2005 Chevy 2500 HD with a 6.0 engine, the temp runs normal when I am not towing, between 180 and 210. When I pull a trailer it all goes south. I have done all the simple checks for head gasket problems and everthing seems ok. New thermostat, new radiator, only thing left is water pump, am I missing something ?

 

Is your cooling fan working like it should? Might get an infrared thermometer and verify the temperature of the coolant and the transmission cooler lines. It's a safe check, but I'm guessing that your cooling fan isn't working right. Personally, I've been towing my boat lately and my temperature does exactly as you describe, except my fan kicks in and all is well. I'm pretty sure a Chevy almost doesn't need its fan when it isn't working lol