It's not an RV part, nor do most RV dealers know much about chassis repairs. Besides, the Spartan chassis was built in Michigan, using components from regular trucking industry sources. I recommend avoiding an RV dealer for any type of chassis repair, unless they are big enough to have their own specialized chassis shop.
If you have the Spartan part number for the bag, try calling around the truck repair places (preferably one that specializes in springs and suspensions) and see if they can match it. Take the RV there if they have it or can get it. There should also be heavy truck parts places that might have them, e.g. a NAPA store that handles heavy & medium duty truck parts as well as cars & light truck.
Use the Spartan Service Locator to find shops that can handle the chassis. It doesn't have to be an authorized Spartan dealer, but it helps to have a shop familiar with the chassis.
http://www.spartanchassis.com/cps/service/service_centers.asp