Ram/Cummins still can be purchased with a six speed manual transmission. The engine puts out 660 foot pounds torque, compared to the Aisan auto trans that puts out 865 or more foot pounds torque. As to the 3500 dually, no brainer, get it. Truck campers will fit the dually with the add on dually extended front jacks. That should not be an issue if you really want to haul truck campers.
Some of the early Rams with the Cummins have now exceeded over one million miles use with the engine never taken apart. Pretty impressive for a pickup.