... ... ... ... . "ST" tires have a higher grade of materials and construcation to meet the higher loads... ... ... ...
Sorry, you're going to have to back that statement up with proof. I believe it is simply not accurate. ST tires "claim" to have better resistance to damage from UV rays. I've never seen any information provided by tire manufacturers backing that claim nor have I seen any evidence proving it. They still deteriorate and should be replaced after 3-4 years as you stated above.
The load rating is molded into the sidewall of all tires. If you compare an ST tire similar in size to an LT (light truck) tire they will have the same weight rating.
The only ST tire I am aware of that claims a heavier weight rating is the cheap ChiComm Mission tire which claims a weight capacity of 3500# (approx) in ST 235/80R16 LRE. I have never seen an ST tire that claimed to have better construction. I do not believe they are constructed differently unless they are constructed more cheaply.
The best trailer tire made is a Michelin XPS Rib with all steel casing. In size LT235/75 R16 LRE Michelin only rates them at the industry standard of 3042# at 80 psi. If any tire is better constructed and made to carry a heavier load it is this Michelin but Michelin doesn't claim any higher weight rating.
NuWa started using Mission ST tires on some models a year ago and I requested that my ordered unit have American made tires and offered to pay the difference. I wouldn't use Mission tires on my lawn tractor.
It is worth considering here that tires made in Communist China, which includes most if not all ST tires, can probably claim any weight rating they wish. I don't know with certainty but doubt that their tires undergo any standards testing by any US government agency. My guess is it is probably a case of "buyer beware. "
The larger, heavier, more expensive fifth wheel trailer brands are equipped with LT235/85 R16 LRE tires which are rated for 3042 pounds at 80 psi. The heaviest of all are equipped with LT not ST tires in LRG which are rated at something like 3700#.