It's hard to offer good suggestions without really understanding what's going on. Probably a good truck spring and chassis shop could give you a better opinion than I have any hope of doing (and, for that matter, that would still be very much true even with my examining the rig in question). However, it does sort of sound like you're hitting the bump stops, in which case stronger springs and/or air helper springs or similar aids should improve things. It's not out of the question that you may have a broken or worn out spring, too.
I do have ride-rite air helper springs on my class C. They do make a noticeable difference (improvement) in its composure in going over bumps. It's not a miraculous change, but it is enough that I'm happy to have them. If nothing else, they do help keep the ride height at a reasonable level (and so give more possible suspension travel when needed).