Don't even waste your time with a gasser in a dually with the loads and trailers you pull, especially at altitude. Especially if you plan on selling it, because very few will want a HD dually with an anemic engine. Resale will be terrible.
Seems like you'll keep the truck a long time as you're rockin a 25 year old truck currently. Another reason to get the right truck the first time.
Mileage on my old 6.2 F250 4x4 3.73 company truck was average to poor. 11-12 mpg hiway commuting winter vs summer, empty truck, 65-75 mph mostly hiway. 8-9 mpg pulling a 4000 lb enclosed trailer with 1000lbs in the bed, long road trip, mostly thru Canada in early spring so mountains, frost heaves, some 75 mph but avg around 60 mph I'd say. Similar load maybe less than a camper pulling a toad. Plus unless you like 2nd gear wound to the beans on every hill like your old 460, get a diesel.
Plus if you're gonna keep the truck forever, the initial cost premium will only be a few hundred bucks a year spread out not including inflation.
Diesels are more maintenance sensitive for long term use IMO but just as reliable as gassers.