So I’m trying to understand the tow haul mode on my soon new to me truck. Ho asin. 3500.  How low will it go in miles per hour before the braking stops 20 ? 30? Miles per hour. Please explain how it works thanks 

I'm not sure. Typically the exhaust brakes on autos only work while the torque converter is locked up. I'm not sure how much you'll get I've not had a chance to play with auto newer generation yet.

 

Now on a manual transmission will brake to near nothing just keep downshifting. 

New truck came now I understand how the brake works    It’s just really awesome. Works great 

It will run down to around 18-20 mph, and then the TQ converter unlocks and the EB disengages. 

Being I don't have any autos to play with typically I've never knew. Thanks @AH64ID