I'm replacing 5th wheel trailer tires now because I lost the back left one recently. I figure the other tire got overloaded and would probably go boom soon. So I bought two new ones. Then I got to thinking and I'm going replace them all for peace of mind. My spare was really old so I bought 5 new tires in all, maybe I'll get another 3 years 7 months of trouble free running.
But I'm dealing with ST's and they don't do well with age. The one that went boom still looked great on the outside. But the trailer only only had about 900 miles on it this year, lots of sitting around.