I don't have much power while towing why?

What is your question?

I have a 2002 GMC Sierra 2500hd. It will run 70 mile an hour down the road beautifully until it's hooked to a trailer. As soon as I hook any decent size load to it it's totally dogged down. Like I'm talking pulling hills at 20 miles an hour if I'm lucky. And it's overheating when while pulling that hill. Temp goes right back down and pulls with decent speed on the level. It has the 6.0 crew cab 8 foot bed. It has had a complete tune up new water pump. The wire for the tow/haul had broke and sent it to be fixed. Which I do use and don't really see any difference when it is engages or not. I bought the truck to work but it's sadly not everything it needs to be. I previously hd the same exact truck but in the Silverado and it pulled wonderful until someone hit it.

What makes this problem better or worse?

Hooking the trailer to it

How long has your 2002 GMC Sierra 2500 HD had this problem?

Since I bought it