First off Green adding air bags to rear of your truck will NOT increase your GVW or payload or anything....they do nothing except make the tow vehicle ride level. Anyone who tells you otherwise is someone to run away from and I have air bags in my truck. Yes I love them.
Increasing your rear end ratio to 4.10 would also be requiring the front dif to be changed also to the same ratio. This might require new springs or coils depending on the configuration of your truck and the upgrade. 4.10 gears are going to increase your fuel consumption, 3.73 gears are an all around nice set for both average fuel and pulling ratio. Upgrading to 4.10 might also require some engine work or the injectors to compensate for change in gears, I would suggest asking your local shop and see what they say.
If your upgrading you might want to consider duallys if your doing all that work however that would mean a different box and wheels all around. Plus other things.
One of the new guidelines I am hearing at our local RV dealers is that 1/4 of the 5th wheel or trailer weight should be considered as payload, ie: 5th wheel weighs 20,000lbs 1/4 is 5,000lbs of payload your truck needs to be able to haul...note I said guidelines..! Guidelines have a habit of becoming law.....at least up here.
Having all that to think about my next question/concern is...you got that big rig rolling, can your truck safely stop it..?
All things to think about, in the end, it your decision on what to do. I hope that helps you out, my post is not meant to be judging just things to consider..!
Glen & Robyn (A.K.A. Puff & Crickit) Full Timers..
2006 F350 6.0 with 3.73 gears, 4X4, Extra-Cab Long Box, Oil/Tranny/P.S. Coolers. Coolant filter. SCT Livewire.
2011 Cedar Creek 36RE, Auto levelers + too much to list..!