What does your owners manual have to say about ;exhaust brake', 'tow/haul mode', 'cruise control' etc.? Lots of info about your particular truck in that manual.
My exhaust brake is aftermarket....I push a button on dash to activate control then when I let off of go pedal the exhaust brake closes creating backpressure which slows vehicle down. With it and in tow/haul mode I can descend a 6% grade holding 45 mph towing 14K 5vr and not have to use brakes.
Sometimes I even have to get it a little go pedal.
Push the button....try it out. You won't hurt anything.
Try the tow/haul when not towing/hauling anything
Try the exhaust brake..towing or not
Push the dang button