I pull a 11200 lb 5th wheel rv trailer and several GN trailers up to 16k with my '03 Dodge/Cummins NV5600 6 speed manual 3.73 axle with the Jacobs exhaust brake.
Its a dream machine for towing heavy trailers however not everyone can use a manual and make glass smooth up and down shifts. After a while you can make those smooth shifts.
The Cummins from '03 and up uses the zero throttle launch system. Simply let the clutch (keep your foot off the go pedel) out and the Cummins rpms comes up which eliminates stalling on any grade. With the deep 1st and 2nd gear there is no need to slip the clutch....ever.
If its flat I take off in 2nd gear empty of pulling. When empty I go with 2nd....4th ....then 6th. Pulling a load I run the gears.