When I was truck shopping 5 years ago, my priorities were a capable, (towing, payload, w/ 4WD) low-mile machine with no rust that fit my modest budget. When an '03 2500HD w/ 6.0L showed up on craigslist with only 88k miles and no rust (from new mexico), I had to jump on it for 13.5k. Probably still worth almost that much today even with 112k miles now.
The 6.0L is arguably the MOST RELIABLE engine ever put in a pickup. Seriously. My earlier year vortec 6.0L only has 300 BHP, but you know what? It still moves along just fine, and pulls 10k lbs without much anxiety. My 4L80 trans is the main limiting factor for obtaining decent mpg. A 6, 8 or even 10 speed would probably add a few mpg in daily driving. I get 12-15 unloaded. 9-10 when pulling my 5k lb camper.
I would think a 2016 6.0L could get mid teens all day long.
I saw OP lives in Louisiana and first thought was, ok, 2WD probably ok. But then wait, don't you have MUD on your farm? surely.