Your leaf springs should be carrying the majority of weight and the bags just help level you out. I carry the AF811 on a 3/4t with upper and lower StableLoads. My bags are only inflated when I tow my enclosed trailer which weighs about as much as your horse trailer. I have OEM upper overload springs and anti-sway bars. The shocks and wheels are upgraded on mine from the lower capacity OEM.
Start by adding StableLoads to your rear and see if you can carry the TC with minimum air in the bags. This should be stable - If not, add anti-sway and better shocks. I would get an actual weight measurement on the rear of the truck - You may find that you are pushing the tire and rim limits of your current setup once you add the hitch weight.