Get the diesel. While people tell you the gas truck will "get the job done just fine", I disagree. If you are truly going to tow HEAVY, be prepared to listen to that engine rev 2500++ rpm, and if your pulling grades on the highway, 4000 + rpm on the highway. Gas trucks can tow heavy, but they work HARD, and the transmission is constantly shifting to try and maintain speed. Whatever extra you pay for the diesel option upfront you will get that money back when you sell it. Look at the price of used diesel HD trucks, they definitely hold their value much better than a gas HD truck.

I had a 2013 Ram 2500 with a 5.7 Hemi before my current truck, a 2016 Ram 2500 with the Cummins. I hated towing with the Hemi, the truck would not make good power below 3000 rpms, sucked towing on the highway. The Cummins on the other hand, that engine loves to tow. The low-end power is huge!

If you go with a diesel, be aware that only GM is now using a fuel pump that isn't the Bosch CP4 time-bomb. Ford and Ram still use that pump. GM switched to a Denso pump in 2017.

Why no mention of Chevy? Just wondering.

I have hauled in many different pickups of many brands. If you are hauling heavy, nothing beats a 3500 long bed IMO. The stability is noticeably better than in a shortbed SRW. The extra wheelbase and the extra contact patches of the two extra tires makes a big difference.

Regarding driving a 3500 Dually empty, it is just ok to me. I will lower the tire pressure to get it to ride a little softer. It still drives a lot better with 1000lbs in the bed than empty. Daily drove a dually for 3 years. While the ride could have been much better I found it really didn't bother me that much. The harder thing to get used to was the long wheel base. Parking and sharp turns were not always fun.

Currently I have 3 Chevy Duramax pickups and one Ford F-450 Stakebed diesel. If you are towing heavy, NOTHING beats a diesel IMO. The torque makes a night and day difference.

Had an ‘89 F350 diesel dually flatbed. International, naturally aspirated with auto trans.
Firm ride. Good engine. Driving with an Empty bed and on anything wet it could be like a pig on ice. The rear was light. Total bed load, according to the factory manual was 3000 lbs.

Now have ‘95 Dodge Ram 2500, Cummins, manual trans SRW, 8’ bed king cab. Very comfortable ride full or empty. Great engine, love the manual trans. NOT 4WD. Average 20MPG around town. Run truck with 10ply tires at 55psi. Ride is still good. According to the factory manual, vehicle can carry 3000 lbs - SAME load as my older Ford.

I agree with the earlier suggestions of single rear wheels vs. duallys. Single is better way to go.

Logically - same load on dually vs single rear tires:
4 tires split the load 4 ways. 2 tires carry have twice the weight on that rubber patch on the pavement.
Which will slip sideways sooner? The dually will slide first. High pressure tires = hard tires = less grip. The Tires should last twice as long and since there are 4, cost twice as much.
Is 55 lbs a lot for tire pressure? Not in the 10 ply I use. I just changed my Cooper tires after 90k miles (yes - ninety thousand miles!) They were not bald, they ran well and still passed inspection. Good tires.