I'm not familiar with the Chevy tow haul mode. But my GPS 2013 super duty will down shift evey time I hit the brakes until it reaches the point when the next gear will cause it to red line.
If the hill you are going down has a truck speed limit I would suggest that you go that speed. Just pick the best gear to hold u back with min use of the brakes and not red line the engine. High rpm are ok just bury the tachometer. Probably going to be 2nd or 3rd gear.
Hope that helps.
Tho I do sense sarcasm I. Ur post