I pull a bigger camper with my Dodge 2500 with the firestone bags. I only run about 23 lbs in them. I think 100lbs is the max. Thing to remember is you already have a suspension holding the weight. You don't need the airbag to take it all or you will have a crappy ride. Honestly, if I had a 3500 I would not buy the airbags for hauling that trailer. You simply do not need them.
Edit: Just looked them up looks like 1 lb of air supports approximatly 40 lbs. So running at 23 lbs after the suspension takes part of the load I have an additional 920 lbs of support. Running at 50 lbs would be way too stiff and make for an uncomfortable ride. I guess it depends on the condition of your existing suspension.