Just because a truck sags with weight doesn't necessarily mean a bigger truck is needed.
The '03 and up 2500 2wd Dodge don't come with a aux overload spring pack. My '03 payload rating says 2460 lbs.
Adding 1800 lbs of Redi-Mix or my current 5th wheel trailers 2240 lb pin over the rear axle has the truck sagging badly.
I had the same trailer on a '01 2wd 2500 Dodge/Cummins with the aux overload spring pack and no sagging even with 2540 lbs of GN hitch weight.
Sometimes folks add suspension help even though the truck doesn't need help.
Also High pressure in a bag can cause a out of tune spring pack with a short compression stroke and a violent rebound and a very bouncy loaded ride.