I came from a gas 6.0. I pull a 8k toy hauler across the Midwest and into the Appalachia's, typically 4-500 miles each way. The gas motor worked but it required some skill in the mountains. If I got stuck behind a slow semi at the base of the mountain there was no way to regain any sort of speed up the incline. I justified it with the "10k for a diesel buys a lot of gas" mindset. Even though I only averaged 12-13 MPG around town 6-7 MPG towing. The truck had plenty of power for flat ground/small inclines and could maintain a reasonable speed in the mountains if I was careful/played the traffic correctly. Honestly my biggest complaint was trying to get fuel with the 30ft trailer behind me. Most gas stations are pretty tight. But the overall cost of ownership was reasonable and it could do what I needed. When I traded it in at 100k i got about 1/2 what I paid for it new.
After a few years of that I picked up the duramax and doubt I will ever go back. The amount of pulling power is a night and day difference. I can easily accelerate up the steepest inclines with the trailer, I can swing through a truck stop to refuel without an issue, and I get 9-10 mpg towing. Are oil changes more, of course. Plus Def usage, and lets not forget there is usually only 1-2 diesel pumps (if any) at most regular gas stations. The entire towing experience is better with a diesel truck, less stress on inclines, less stress over refueling, and farther distance between refuels makes the extra cost worth it to me.
So back to the original question on cost of ownership... Is a gas truck cheaper to own/operate than a diesel. Yes in most cases. but there is more to it than dollars per mile/resale value.