Tow/haul mode is designed solely to protect the transmission by changing shift points and keeping the torque converter locked more of the time.
Don't know about the Tahoe but my Silverado 2500 has a feature called "grade control" and it only works when the transmission is in Auto. Put in in Manual and grade control is disabled. Grade control on my truck works perfectly and unless I have a very heavy load (70% of the GCWR) I don't bother with Tow/Haul mode as this is what GM recommends for my truck.
When all else fails you can read the manual.
