NEW Dodge 2013 Ram 3500 4x4 -> ride is horrible! What to do?

tire over/under inflation will affect the ride and handling of ANY vehicle on the road. It's not a phenomenon specific to trucks. You should have had him air the tires up BEFORE you signed the paperwork to cover your ass. Expensive lesson learned, but I suspect that you had a unrealistic expectation of how the truck was going to handle on the road without a load.