2009 Adventurer 950B on 1999 Dodge 3500 Diesel LB Dually

You have a Dana 80 rear axle. It is an 11klb axle. So tires wheels axle is rated far heavier than the truck rating.
On paper you have about a 4klb payload, so your math is correct.
Leaves the 2 main concerns being frame and springs.
Both will handle a large camper no problem except for spring rate. You may/will need to add/modify the rear suspension to handle 2 tons plus in the bed.
The last concern is the condition of the truck. At almost 25 years old, it could be anywhere on the spectrum. My recommendation only applies if it’s in good well maintained condition.

Biggest difference from a high cg weight hauling perspective compared to newer trucks is the 2nd Gens aren’t a fully boxed frame so they aren’t as torsionally stiff.
It’ll be a little more of a noodle than if on a newer truck. Makes having sufficient suspension upgrades including heavy duty sway bars more important to control body roll.