We have a 24 foot 11,800 lb. Class C on an E450 chassis and run 80lbs. of pressure in the rear Michelin tires.
Like (or worse than) the OP, the pounding in the rear of our rig on an under-loaded chassis from highway potholes and cracks was horrendous - especially in places like the truck lane of I80 in the High Sierras where tire chains, freezing/unfreezing, and big-rig vehicle weight in the winter combine to really ruin concrete.
I waited way too long to replace the stock rear Ford shocks with a variable rate rear shock. These Koni FSD shocks have really added to reduction of the pounding in the rear of our Class C:
http://www.konirv.com/FSD.cfm