[RMcNeal]

I have a 2014 with a custom towing tune. The exhaust brake kicks in and will hold you back on a downhill very well. As said above, if you want to force it you can turn off tow/haul and then re-engage, but I never bother as I usually need the exhaust brake to hold me back on steep grades. If it's very steep, like 7% or 9%, you may want to start down slower to keep it from revving too high.