Slow, the airbag rear end on my 28' AS 1984 was done by ACS and they still support the system. I replaced mine last year as they where dry rotted. As long as they hold air and don't look to be deteriorated they should be good for a long, long time. I think mine where original.
I had mine done by the local truck suspension outfit in Columbia, SC who are an ACS shop.
Not sure what year your coach is but mine came with 19.5" rubber.
1984 28' Funeral Coach
Former Rolling Showroom & PuttLab (now party bus)
"I'm not an expert. But I did sleep in an Airstream last night."