Didn't think I could add anything useful to this thread until I read all the posts. I have an E450 chassis on a small CoachHouse - - Ride was extremely rough so after quite a bit of research, I bought and installed (myself) a Reyco Granning air suspension for the rear end. Totally replaces the OEM leaf springs with large air bags.
Cost about three grand, and took me a couple of weeks to install (I'm OLD). Made a big difference tho. No tricks or magic to the installation but still a LOT of work. Biggest issue in my mind is to make sure ahead of time that you have adequate clearance for all the new components. You DON'T want to have to move the gas tank, holding tanks or genset to get the new system installed.
Will be glad to answer questions if anyone's interested.
rhaag11@charter.net - - aka oldusedbear