The first thing I would do is weigh all 4 corners. If your coach is ability equipped you may be over the weight the springs and axles are designed to carry. If you are overweight new springs will only be a temporary fix. Over time they will sag just like the current ones.
As a side note we had a 2001 Winnebago Adventurer on the20,500 lb. F53 chassis for over 12 years. In that time we put over 100,000 miles on it. The weight was always around 19,000 lbs, and we pulled either a Jeep 4 down or Buick on a dolly. The springs were never overloaded and never gave problems.