Saw an RV forum post about better durability of trailer tires.
The poster said: " Old trailer tires were rated to 65 mph. Newer tires are rated to 81 or
88 mph. That is a big difference, and since i upgraded to Load Range D
and Speed Rating M (81 mph), my trailer blowouts have disappeared. "
Also there is no DOT test for this rating so I doubt that you can depend on all tires of a specific speed rating to perform the same. With no federal regulation for speed performance, I do not see a reason to expect any ST type tire to be capable of running at the stated speed for more than 30 minutes when brand new on a perfectly smooth surface as used in the test laboratory.
When shopping for new tires for your trailer I suggest you "read the fine print". Look at the material list for the tread area molded on the tire sidewall. An ST tire with more than just steel and polyester listed i.e. Polyester + 2 layer Steel + 1 layer Nylon, will probability be more durable than one without the Nylon when operated at speeds above 65 MPH. IMO a "Speed rating" of L ( 75 mph) should be more than sufficient for RV trailer use.