How fast are you towing? I typically run 62 to 75 on most trips depending on wind direction for best fuel mileage and traffic/road conditions.
Speed and heat buildup can easily kill a tire. Most ST tires from a few years ago were only rated to 65mph. Trailer manufacturers tend to use cheaper tires that are rated to do the job but are lacking in durability.
The Goodyear Endurance ST and Carlisle HD get top ratings from many folks.
My Kumho Radial 857 tires 205/R14 are load range D, speed rated to 106mph. So far I have 15,000 miles on them and all is good. These are LT's only avail in size 14.
If your rims are 15", GY Endurance ST. A LT tire like Michelin XPS RIBS will do well for 16" rims.