What is the max tire pressure on the sidewall? For driving empty definitely follow the door panel sticker. For heavy hauling try pumping up the rear tires to their maximum. Converting to an LT tire would definitely help. Maybe separate RV rims for the Dodge with LT tires. This would be a cheaper option than a new vehicle.
Looks like the Citadel came with 20's? Will the lower model 18's fit? Some dealers have "take offs". These are new factory tires and rims that are set aside for aftermarket upgrades for the showroom floor. These are typically well priced. Will the 1/2ton Dodge Truck rims fit the Durango? If so, take offs have around 35 total miles. They are usually priced really well. The truck is more likely to have LT tires versus the Durango P rated. 18's would have the benefit of more sidewall.