Truck in front of the camper

Been running a 2005 8.1L 3500 SRW with a ~4000lb wet Eagle Cap 950 since late 2006. I added Rancho adjustable shocks but not sure I noticed a huge improvement. I'm sure they are better than what OEM shocks would be once worn out. No issue with power even with a 2k trailer in tow. I've only checked the mileage a couple times and it's solidly in the 8's with mixed freeway and back roads.

Only ~70k miles on the truck with a MAF sensor, oil pressure sender and coolant cap replacement so far.

If you are looking for "any camper you find" you will need a dually or limit your choices to ~9.5' campers or less.

Don't have one but always was impressed with the three 7.4 I've had. The 8.1 is a tork machine! Primarily on heavy duty PU. The Vortec can be a tricky ***** but you probably already no that.

https://chevytrucks.org/chevy-8-1-vortec-ultimate-engine-guide/

Has virtually nothing to do with the engine/power and everything to do with the class of truck and tires/wheels/suspension.
An 8.1 would not even be a good consideration imo unless you needed the extra power to tow. And at that, most of the advantage would be in the Alli vs the 4L80 behind the 6.0.
Toss up really dead reliable 6.0 with not as awesome of a trans vs 8.1 which could be great, could suck oil but the best trans.
And going old, it’s less about brand and more about miles and condition.
I’d be looking at older Super duty gassers equally. And 3rd gen Dodges if you find a cherry one. They’d be last on the list for gassers IMO.