07 E450 Class C rear air bags better handling while towing

What is the "loaded for travel" weight of your unit? I see it has a GVWR of 14050 lbs. How close to that number are you when you travel? If you're within a hundred pounds of that max, or possiby over it, your problem is the chassis is overloaded and there's not much you can do about it. It's subject to the laws of physics. You can make the suspension more firm or robust, but that just moves the stress of the weight to the tires and frame. Find a way to lighten it up, and see if that helps. Otherwise, get used to it.