Can you comment on the ride when you were over weight on the rear?
Having a rear axle over-load condition on our 2007 E350 2350 (with our lighter-weighted NO-slide-out floor plan), the ride in back is fairly smooth, yet still rougher than our family sedan. I will always wonder if Koni-FSD shocks all around would help, but I likely will never find out because I am extremely happy with the Bilstein-HD shocks I recently installed.
I "test feel" the rear axle ride when Irene is driving on the open road, and I lay on my back on the floor with my head resting on the floor over the rear axle. I feel the vibrations best that way.
Under our typical load conditions, the ride is a bit harsher than when over-loaded but not significantly so, because we always run very close to the 7800 pound rear axle load rating.
It is up front where I always felt our PC could benefit from a suspension adjustment, hence going to try E150 front coil springs. I hope the results will provide a noticeably softer ride along with a little lower front stance, without a significant sacrifice to handling. If the front end lowers enough for a greater clearance between my roof a/c to our garage door opening, then I will consider adding one more leaf spring in each rear corner to lift the tail a bit more.
Keep in-mind that our 2007 E350 2350, does not have the extra weight of a slide out which lightens the front axle, and our fresh water tank sits against the back wall which teeter-totters the weight even worse off the front and onto the rear axle. Later model years of the 2350 have the fresh water tank placed more forward and more centered side-to-side. So my situation is worse than 2350 owners of a 2010 model year or newer.
About rear air bags. I had them on our first motor home. I had an on-board air compressor and controls by the driver so that while driving, I could add air or let out air pending the driving conditions. More air in the air bags provided better handling which also made a rougher ride. So I am surprised to read that people say to add more air in the rear air bag suspension to make a smoother softer ride.
Regarding the tire pressure. That is one easy area where you can make a difference in the harshness of your ride, but you really need to know your weight per axle to know the right tire pressure. All 4 rear tires get the same tire pressure. The same up front, but they don't have to match the back. Just 5 PSI pressure more than needed will yield a noticeably rougher ride. For reference alone, I run 65 psi in all 6 of our tires, making sure my tire gauge is accurate.