If you're the least bit unsure, replace 'em. I was enroute back to Kingsville Texas and one blew apart, replaced it with the spare...and sure enough 60 miles later the other one came apart (pictured ...
When I bought my trailer the seller stated "new", tires. The build date of the tires was 2008. When I made an issue of the old tires, I think he thought I was trying to scam him. To him, the tires looked brand new. I put new tires on and just completed a 3500 mile trip with no problems, and peace of mind!