This thread is speaking to two different types of axles. A torsion axle that the spring is inside the axle. The spring is basically rubber. More complex than that but that is the idea. The other type is a leaf spring type. It has metal springs of flat metal on the outside on both sides of the trailer.
is close to the year that Airstream made the change from leaf to torsion, the question would be what type do you have?
If you are going to replace the springs or torsion axle, you may want to consider the total load as pointed out above. Get the trailer weighed. Also if you are adding a gray water tank, assume 8 pound a gallon for that load plus the tank and related piping. Then I would add 10% for the total weight for a safety margin.