I know exactly how you feel. I had to to stop renting the Express vans for my large group trips because they often became the puker vans.
The biggest problem is the OEM shocks. They are terrible. Upgrade to Bilsteins or KYB MonoMax, and most of the bounce will go away.
Airbags do not generally reduce bounce. Sometimes they can make is worse if the airbags are too short (reducing spring travel). They are good at leveling, and can reduce some harshness when loaded. I had to switch airbags because my old airbags cause too much bounce sometimes. My new airbags are tapered and have 12" of travel vs 4" that the old ones had.
