In their literature its says 1223 provide a 25% improvement in load carrying capacity 50% for 1211. My factory springs alone were only good to carry about 700 pounds so I figured about 950 with the 1211s. I installed the 1211s and have been happy with the additional support and the ride not harsh or stiff. I still need want benefit from additional support for dramatically varying weight loads potentially up to 1,800 pounds between bed & tongue. Yea can get into HD territory. Truck upgrade eventually.
This is where quality axle to frame air bags are the bomb, for support dampening & rear suspension control. But if you don't want air. The Sumo neoprene Springs that fit in place of the factory bump stops are the best option for additional support. They have two densities for the 1500 rated to carry 1,000 or 1,500 pounds at 50% crush. 1211 & Sumo equals great combo.
That said as you may know with a 30 plus foot travel trailer you would still want and dramatically benefit from a well set up WDH. As stability stopping sway have a lot more to do with axle and tongue weight than a no sag level ride height on your truck.
