To identify an Airide spring for replacement, find the Assembly Order Number (AON) on the label adhered to the bead plate (e.g., w01-358-9448). This number is used for ordering purposes. Do not confuse this with the stamped 8-digit bead plate number. If this is not present, look for the bellows number molded into the rubber bellows and do some cross referencing to narrow down your choices (the same bellows will be used across a number of assemblies).
There is no standard that defines the life of the bag. A lot depends on the age of your coach and the environment that it is driven and stored in. You see many people covering their tires and even their windshield wipers when parking their motorhome to protect them from ultra violet light. Our Air Ride Suspensions air bags are just as susceptible to deterioration. A visual inspection of the bags should be part of your routine. If your coach is 15 years or older, the bags could be showing signs of cracking or flaking, usually at the bottom where the bags flexes the most. If they are, then it might be time to replace them.
You could change out the bellows and save a few dollars. No matter which way you go the whole thing has to come out. That bellows is used in several applications with different end plates per individual manufacture requirements. This is not just used in F/T suspensions with these end plates. It may not be sold w/o the plates.
For the Lifting Tag Axle, these are the ones that hold downward pressure
There are several very good threads with details on how to go about replacing the air bags. There is even a set of “loaner” tools that one member will ship to you for use, these are one of several ways to get to the “hidden” nuts (homemade tools / cut access port / cut bags / etc.) on the front passenger side airbag.