Here is a picture and comment I found on an RV Forum. I suggested that the owner review his understanding of tire construction and that he read the material list that is molded on the sidewall of all tires. The failed tire does not have 12 or 14 "ply". I also pointed out that it does not appear to have a "ply" of Nylon over the steel belts.
Do you know how many "ply" or layers of which materials are in your tires?
The owner said "After many problems with ST 12 ply I have moved to "xxx" brand 14 ply and no more problems. I drive to Mexico and back every year 9,000 mi."
I pointed out the problem with thinking that there were actual 12 or 14 "Ply" in the tires as that term lost its meaning when tires were switched from "Bias" or "Diagonal" to "Radial". I strongly recommend that ST type tires include Nylon or similar material as "Cap Ply" over the steel belts. This can help lower the Interply Shear stress forces found in tandem axle trailer applications.
If you do not know what Interply Shear is then I suggest you review this post.
The tire failure probably had a contributory cause of impact damage from the poor roads in Mexico.
Here is my post on the scientific study of impacts and tire failure showing a 100% correlation.