This is just my opinion of course,
we have a 38ft montana fifth wheel, and then pull a 20ft walleye boat behind that so we come in around 18,000lbs and change for that particular rig, as well have a 40ft gooseneck that we use for the welding business but it also hauls equipment at times and it weighs in close to 22-24,000lbs. The last 3-4 years we've been pulling that with a 07 ram 3500 dually 5.9 w/ G56 and well, yep that truck was out classed, and you had to really be on youre game when pulling hills or decending. Previously we were running a 15 ram limited dually which to date has been my most favorite truck of all time that thing was a beast.
after so many years of hauling what i would consider moderately heavy loads for camping and the business for ME id never be without a dually in my scenerio. Having said that iam a 100% advocate for being overkill to a minor extent. I feel having too much truck when towing longer/heavier fifth wheels or even goosenecks to provide an extra safety factor is smart. Again not to bash the 6.4, but ram saying itll haul 16,000LBS is fine and dandy but i seriously feel thats a real world on a perfectly flat road with no head wind lol. Another thing you should be seriously paying attention to is payloads. On a 14,000LB camper which is about where our montana sits at 14,600LBS GVWR we have a pin weight of about 3600lbs. So youre seriously going to suck up the capability of a SRW 3500 real quick. Which on a side bar makes no sense to me, why these manufacturers say a SRW 3500 can haul 20,000lbs when it most certainly cant handle the pin weight 20,000lbs brings with it. I seriously researched moving to a 2022 srw 3500 for our camping needs but in no scenerio would it fit for our montana/boat combo payload wise unless i opted for a reg cab work truck long box lol. It is seriously sad how little payload a well apointed laramie 3500 srw has. So i scraped that idea, and decided to order a 5500 laramie to facilitate as our new welding rig and family vehicle for hauling camper etc, and frankly to give myself peace of mine to never have to worry myself with payload numbers again.
anyway again just my opinion 14,000 and up should absolutely be dually territory all day. From a payload reserve, overall stability, and towing performance perspective. But even at 14,000lbs with a dually and correct pin weight on the trailer youre still only going to have MAX 2000lbs reserve left for fuel, passengers, toolbox, fifth wheel hitch etc You would be shocked to see how quickly it disappears. In canada anyway, if your overloaded and get into an accident you have zero insurance.
As far as the cummins goes, frankly i think you would be seriously disappointed with not going with a cummins versus a 6.4. the towing comparison is not even in the same universe. Again just my opinion, but for a more efficient, safe, reliable tow. Id set that mark at 10,000LB and up would be cummins all da