Not sure I agree. If the rear is heavy enough that the rear is sagging, adding a leaf to bring it back to factory spec should not make for a harsh ride.
Yes, the springs have become weaker over the years. An added leaf should restore their deflection per pound/force.
Of course if the IS something that concerns you, another option to restore ride height without adding to spring rate is to have a SPACER added. Did that on our Safari DP with leaf spring suspension. Worked just fine. Note: Did not do it because of sagging axle, but side to side lean due to all heavy things on one side of the coach.
