If you are running empty with max tire pressure, especially with 20 inch wheels, that will contribute to a stiff, busy feeling ride quality, especially at low speed, and i don’t think shocks can fix that. You might try adjusting tire pressure if you haven’t already. I’m running my new 3500 empty during break in at 55 front 60 rear and it is noticeably better than the 60/80 max the dealer delivered it with. I have the camper torsion bar option and 18 inch wheels. When the 2500 lb slide in camper goes on in a couple weeks I expect the ride will be pretty nice.
I suspect a set of Bilsteins are in my future, though.