I think in Francesca's post above, she hit the nail squarely on the head as to why the RV ( and other trailer manufacturers ) companies use ST tires: LOWER COST per tire.
If you are the top dog at a trailer company, you might tell your employees, "find the parts to build these trailers, that meet all the standards for the use of this trailer. And oh by the way, find the parts that meet those standards, at the lowest possible cost to us".
Congratulations....welcome to world of ST tires. Cheap, and meet the minimum standards. We therefore can say they are "adequate", so to spend more money on more expensive components simply robs us of bottom line profit.