I wouldn't buy anything newer now a days. I would find a low miles original, nice, 7.3 Ford diesel and do some mild mods for the camper. Or go for the older 12 valve Cummins. Better to go for a standard trans in most cases.
I think the 9-10 year old Duramax Chevy/ GMC are good too. I like that Allison Trans.
Basically try finding something EPA has not got their hands on.
You could have a very nice truck this way gong to the right body shop, pic the right color theme you could have a very nice truck that not only looks good, but, built better they can be easily be diagnosed and easily repaired with wrenches.