Take it for what it is worth. I am a dude on the internet. But I also hold Cummins, Duramax and Powerstroke Certifications from the 3 manufactures.
Hands down best tranny in the business is the Allison, however the Duramax is severally underpowered when comparing to the cummins. The trucks do not tow the same at all. You cannot miss an extra 150 FTlbs of torque.
the Dodge 3500 with HO package would be the truck to get. the rear suspension will hold whatever you throw on it, the system is self leveling and the Aisin transmission is the same one they are using on box trucks in asia/europe/africa. It is a true medium duty transmission in a LIGHT duty truck. Good luck killing it. Also from a MPG standpoint I have several customers with new Dmax's and Cummins, and the report is pretty even on MPG maybe leaning towards the dodge, but remember you are also doing so with more power.
IFS vs. Dana 60. sure the front Differential of the GMC platform is the same ring gear size, but it is NOT the same in loaded strength. The front steering on the GMC's leave a lot to be desired and can fail in terrible places. The pitman arms and idlers are known to fail and having a ton of riding weight on them does not make it any better, especially as it articulates over rough terrain. Ball joints, upper control arm bushings, tie rods, pitman arms, not uncommon to see these repairs repetitively for the GMCS that decide to be used as capable 4x4's. The main point that kills it for me is when the steering fails, you are most likely not going to be going anywhere and if you are moving when it does it will take you right off the road. There are aftermarket companies that make double shear supports to prevent this. I would recommend them ASAP before you put a camper on it.
If you ever decide to go with say an arb in the front on either rig, you will also come to hate the IFS cause locked IFS tend to go into a death shake when trying to climb anything, this is caused by the suspension articulating and pulling/pushing the tie rod, again, not good period, and also does not bode for a good long service life of steering parts. try channeling 750ftlbs through a cv that is flapping back and forth because of tie rod deflection.
Final point, also think of it this way, when you lift either rig if you do down the road, you actually place more leverage on the IFS suspension while doing so. again, not the greatest for longevity on the IFS, but on a solid axle rig, you do not leverage the axle, ball joints, steering components.
Lance campers are heavy, largely because they are built really well. But on a 3/4 truck, even with bags and a hellwig swaybar I have one family member that did not keep his 961 Lance on his 06 Ram 2500 4x4. He hated the "boat" sway. I personally felt it was capable, the truck lacked for nothing in the power department, but you could tell the truck was carrying a massive amount of weight. I have a customer with practically the same rig but a 3500 and there was a large difference in how it rode with just a sway bar in the rear and no bags.
Good luck either way! As I see...you already got a RAM LOL, well congrats!