Some good advice here. I'm of similar opinion of burningman and Vinsil.
Have put plenty of miles on my shortbed 3/4 ton truck with the AF camper that is by all accounts over 4klbs loaded.
Of course a dually would be more stable and ideal for a heavy camper, but with some select mods I'm comfortable driving it nearly anywhere I'd drive empty, save for steep slopes off road.
Part of it boils down to, one either understands how a vehicle reacts under various conditions and can handle it and is comfortable with those conditions, or isn't.
Given the choice I'd go 3500 because who doesn't like more springs for the same price....
If I'm ever able to plop a camper on the truck for extended periods of time and that is the truck's primary use it'll be a dually if it's a big camper.