Lots of subjective information here and perhaps a bit of misinformation.
I'm newer to rokslide, but have done quite a bit of research on trucks and campers in recent years and also haul a camper everywhere I go in my F350 (work from home, and wife has a smaller car).
Unloaded, 20mpg is a good starting point for most diesels. 25 is highly questionable outside of perfect circumstances. With a truck camper it's really hard to get better than 15 unless you are drafting a big rig or going 47 MPH or thereabouts (sweet spot for rpms and wind resistance with my 3.53 gears). At 60 mph, I get 14.5 at best. And it drops sharply from there. Over 70mph takes it down toward 12 or 12.5.
You'll hear lots of EGR/DEF horror stories. Some are as bad as they sound. Some are a byproduct of people thinking no harm comes from running an aftermarket tuner or delete program. Some people just don't read the manual, which makes the driving requirements and best practices pretty clear.
All that having been said, in my 2017 f350, I've not once smelled diesel (kind of a bummer actually), seen smoke, or even been aware of the regen process apart from being able to smell it at the end of a long drive.
Consider just how damn many of these motors are on the road and how abused they can be. And then consider you're only getting a very large sample of a very small cohort of people who've had serious issues. I almost let this dissuade me from getting my truck. The Ford 6.7 specifically has been in service with minimal changes since 2011! That's ultimately why I decided on Ford, although all 3 are doing good things these days (as long as you read the manual!).
Don't worry about cold starting or block heaters unless you're routinely under 0 degrees Fahrenheit. That's in the manual, by the way. I'm routinely camping on single digits. It fires up instantly, always.
With respect to payload, if you go 3/4 ton, get a gasser. If you listen to nothing else I say, listen to that. A 3/4 ton diesel has an appalling payload. If you're only ever going to pull a travel trailer, slightly different story. More to the point though, the newer suspensions are so good unloaded, there's not much reason to avoid a 1 ton.
If you're going to haul a truck camper, get a dually. I wish I had, but I'm making the SRW thing work without issue (only issue is I can't upgrade to a bigger camper with this truck).
Definitely calculate upfront cost and maintenance and then consider if gas might make more sense. It'll definitely haul more in the bed all other things being equal.
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