Originally Posted by MrPiper
Air bags do not increase the payload capacity for your truck. They keep the rear from sagging under the weight and help level the ride when towing, but the payload capacity is what it is for your truck and is not affected by air bags sadly. Says so right in the instructions for the air bag kit.
+1 on this, the truck is made for X payload, adding aftermarket products may help in carrying more payload but the truck is not designed for it and issues may arise from overloading the truck, the extra load may also induce safety issues and kill your trucks warranty. Best bet invest in a trailer that can carry the extra loads and keep your current truck or buy a bigger truck that has the payload capacity of your needs.