Weigh the four corners. If a side or corner is heavy and that's where it sags to, then spacers <<>> adding a leaf to that spring/side would make sense. But if it's well balanced side to side weight wise, then time for new springs.
At least that's my logic. If there's a flaw in it let me know.
Or first Class C sagged in the rear. We corrected that with "repair leaves." Those are a generic part available in different Lengths, Widths and Thicknesses. They have to be sheared to length and have the center hole punched in them. A good spring shop can do both. Then it's a matter of removing the U-Bolts and breaking the spring stack down to shuffle the leaf in. Then of course reassemble. The center bolt will have to be replaced and it's recommended to replace the U-Bolts also. The repair worked out very well, but rear was then a little high. I think we used 1/2" thick leaves. If there'd been 3/8" the coach probably would have come out level front-to-rear.