[ack fia]

I tow my 18' enclosed trailer with a 2006 GMC Sierra 1500 Vortex Max. The only difference between it and your Denali is your AWD where as I have selectable 4 wheel drive.
Your trailer hookup is a breeze. If you don't have a trailer package there are still wires to the back of your truck. They are tucked up under the brake booster in front. At the back, probably on the drivers side frame rail, you will find them folded back and taped. You can hard wire per your controller instructions.
If a trailer option, the brakes can be hooked up with a small loom that you can buy. I use a Prodigy brake controller and that loom plugs right into the junction box to the left of the steering column under the dash.
To "power" the brakes you need to go into the main electrical box under the hood in front of the PS booster. There should be a "blank" fuse that needs to be replaced with a 45 amp plug in. The fuse instructions tell you which it is.
I use a weight distribution hitch and it tows like a dream.