Unfortunately, after many years of towing, I have had more than my fair share of blow outs. My rule now is to replace trailer tires when they turn 5 years old (verified by the DOT number on the sidewall).
As was alluded to, it is less expensive to replace the tires early than it is to replace the tire plus the fender/skirting/damage from a tread separation.
And, since your next post might be a question as to what tires to buy, I will put my plug in for the new Good Year Endurance. I have LT tires on my 5er, but I switched to the Endurance on my 7000 pound boat last year. It made a big improvement in the way it tows compared to the OEM junk tires that were on it.