There is a miscommunication somewhere. The Airlift 5000 is an add on helper spring. Your suspension is an air spring not an add on helper spring. Here is a link to help identify your air spring/bag. Identifying a Firestone Air Ride Spring, Air Spring Tips | SD Truck Springs
The Airlift 5000 handles 5000 lbs. over load. Your rear end weighs closer to 10,000 lbs. If you have the manual copy the picture of the suspension and email it to SD. If you don't have the manual I can send you a pdf and you can print the page and scan it. I am on the road so I don't have a printer or scanner with me.
The system on my coach is made by American Carrier Equipment but your earlier coach is a different set up and I don't know the manufacturer. The closest example I can think of is a Peterbilt Air Leaf. Peterbilt places the air spring between the rear of the leaf spring and the frame. Basically cutting the rear of the leaf spring off and replacing the shackle with the air spring. Your system cuts the rear of the leaf spring off but adds a trailing arm from the axle to connect the air spring to the frame. I don't know how to better describe it in words but this might help the guys at SD to understand what you have without pictures.
The best would be to identify the numbers on you air spring and they will know exactly what you need. The link above will show you how. I have the numbers fo my system but I don't know if they are close to what you need. Anything I can do to help let me know.