We bought a 1984 Class C on a 1983 E350. It sagged and it swayed. None of the leaves was broken, but we heard that "re-arching" them doesn't last. So we added a "repair leaf" to each side. It made a huge difference. On the downside, the added lift in the rear, meant it always sat nose low. That doesn't matter on a passenger vehicle or a cargo truck, but on our RV it meant that we always had to run the front wheels up on blocks to be level on a level site.
Lift in back is nice, cuts down on dragging. Just don't get carried away.