Choosing a used truck

I'd guess my net environmental impact is just now lower than US average because, despite my fuel consumption when moving my home, it's tiny, energy independent, and it doesn't move far or frequently. I don't really know. I just don't want to be judged unfairly, particularly when seeking help trying to do it even better.

I want to downsize my truck for cost and fuel efficiency. I've had this truck, my first, for a year, 3.5k miles. I've towed the trailer a short thousand miles without incident and including city, highway, and interstate.

Current setup:

'19 Chevy 2500 6.0L 4WD

Hitch towing ~2.5 tons GVWR (14', enclosed, tandem, brakes)

Getting 8mpg @ 70-75mph

Next truck budget is $10-20k. I'll keep it probably until the frame rots. I'm planning on replacing shocks & wearable steering components, am not averse to some work.

Should I target a 1/2 ton gas (leaning Ford 5.0L 4WD), a different 3/4 ton gas (which and why), or a 3/4 ton diesel (leaning Dodge Cummins)?

The paper numbers say I should get a 1/2 ton gas. But, my more experienced friend thinks I'll be a lot happier spending more for a diesel because diesel engines can last a long time, it'll at least double my fuel efficiency, and it's a little extra overkill for an easier tow.

I'm open to all informed perspectives. What's my best plan and why?

Edit: I kept the Chevy 2500 6.0L because the local market didn't support transition on the sale side. I also bought a '98 Dodge Cummins 12v diesel that needs work. It'll eventually replace the other truck.