OP, realize the banter about trucks has just started over again for the 1000th time with the crew on this forum.
You're on the right track though. What jimh said is about spot on.
1 ton srw truck will do the job with more upgrades than a dually would need and not be 100% "legal" everywhere, if that's a concern for you. It's not a concern for some, myself included.
A 3500 dually is the sweet spot for hauling that much weight. Minimal to zero upgrades needed but minor rear suspension upgrades maybe preferred.
450/4500 is also fine but overkill. Not bad, just more than needed. Depends what else you'll do with the truck after the year long trip is up. 450 will be a lumber wagon for anything other than towing or hauling heavy.
Gas v diesel? Again, flip a coin. Do you want the luxury of more power than you'll ever need to haul a TC and 30% ish better loaded fuel mileage at the initial extra expense of the diesel? (Resale will net most/all that money back if the truck is well cared for)
Or do you want the perceived simplicity of the gas engine and the lower priced truck with worse fuel mileage?
That is a run your numbers thing.
Now for my opinion......I run a heavy camper on a srw truck and have no problem with it, but it's not on the truck full time. If I had a dedicated TC hauler for a 4-5klb camper, a dually would be greatly preferred.
But I wouldn't even consider a gas dually, A. Because it's kinda embarrassing to have the biggest baddest truck with the littlest weakest engine, lol! And B. Because the resale on a big crew cab dually 4x4 truck with a gas engine is abysmal. You limit the number of perspective Byers to basically only someone who's buying a dually for local "work" and doesn't know what they're doing, or the dedicated camper hauler, of whom are very few.
This will raise eyebrows with some of the codgers here, but just do a little research, look at used trucks and their prices and how many are gas and diesel in the big HD pickups.
Buying a gasser dually is like buying a pink truck. It may do the the job, but ain't no one going to love it but you!
Good luck, have fun shopping!