After my first set nearly stranded us on the pass when we went over the mountains to pick up our current heap, er, car, I replaced all four with Kenda Loadstar load range D tires. So far I'm pretty happy with them. Trailer is a 27' enclosed rated for 10K gross.
I think on the first set, it was either A) they were junk, which is entirely possible, or B) when I first started towing I was probably pushing them a little too much (driving 75 MPH) and I think they got too hot and the rubber broke down. There was cord visible in patches on 2 or 3 of them where chunk of rubber had come off.
Now I drive at 60 MPH max and that's it. And +1 on checking the pressures often and keeping them at the max listed on the sidewall.
Perennial Losers
Mazda 787b aka '75 914 - Org Choice @ The Ridge, 2015; IGS @ TH, 2018
BMW M1 Procar aka '80 RX-7 (retired) - Zero awards