Why diesel trucks?

For me, it’s a diesel to be able to more easily pull the mtns and I’m assured no issues towing anything that I ever might want at a reasonable speed.

A DRW truck hauling camper means it's that much easier to drag a big heavy trailer behind you if not now then someday.

With a SRW, diesel is probably overkill but many prefer it anyway.

Without towing, your camper weight is within the range of what a V6 could tow.

For just a TC there is no good reason to get a diesel, as you have noted it actually reduced the mathematical payload of the truck.
I also use my dually for other truck purposes like towing equipment trailers loaded with excavators around so as a dual purpose vehicle diesel makes sense.
One reason many people have a TC is they can tow large trailers behind them. I did that with my boat behind the TC also. With my fully loaded truck, camper, and boat I was around 18,000 pounds.
Also, a diesel dually is much easier to find on a dealer's lot and much easier to sell than a gas model.
If you are only hauling the TC or towing small trailers and you intend to keep the truck a long time I would get a gas engine.

I bought an older low mileaged mint DRW to haul my beast around for less than 1/3rd of what a new one costs. Not many if any older Ford DRW gassers around, so diesel it was. Pretty much a diesel only game with Ford F350 DRW’s in any year really. I have a company rig I make my living in, hard to justify a new deticated camper/boat hauler with a few beer runs sprinkled in for around 4k miles a year. BTW, my work telco rig is a gas powered V10 2017 F450 DRW. I get 6-7 mpg. Glad I don’t pay that gas bill! My Diesel with the camper on it gets double that and the power is nice to have.

With the new gasoline engines coming from Ford, Ram and GM there is no real need to have a diesel engine just to haul a TC. If you plan on towing a large trailer then a diesel may be the better choice.

You buy a diesel for the performance. THe diesel makes it power at lower RPM's.
In the end the diesel is providing more low end torque/power creating a more relaxed experience. The engine is not spinning as fast or revving as loud vs. a gasser. The transmission is not shifting or searching for the right gear.
The diesel is simply more capable when at the extremes.
If your only towing a few times a year on flat ground a gasser will be OK.
However if you tow regularly especially in hilly or mountainous terrain you will want a diesel.
The big three will all have a larger gas motor in 2020 which may change the equation going forward

It's all about the mountains for me. My experiences towing through the Rockies and Sierra Nevada mountains has been lackluster at best with a gasoline engine. For me, a diesel is well worth the expense and reduced payload for these situations.

Gas trucks do not have more useable payload.
The extra weight of a diesel engine is all on the front.
The camper weight all goes on the rear.

Coming from a under powered 5.8L gas truck, I was not convinced that the modern 6.2L gas trucks would perform all that much better than what I already had. As an example with the camper on I had ~200-225 mile range with 34 gallons of gas.

As it ended up the decision came down to special order a truck or buy the last one off the lot. Around my area, all the dealers lots were chock full of Crew cab short bed trucks, but it was a challenge to actually find a super cab truck with a long bed. Since I was already leaning diesel it was a no brainer that the last super cab Ford in my area was a 6.7L PSD.

Personally, I like the power and fuel economy.

- Mark0.