When the RV trailer manufacturer selects the ST tire for fitment on any of their trailers and certifies that fitment on the trailer's certification label it becomes the standard for that trailer or line of trailers. At the trailer manufacturers discretion other tires such as the LT can be options if so stipulated in the trailer's owner's manual.
I don't think you will find any major tire manufacturer that will dispute the trailer manufacturer's discretion to use tires designed for trailer service. I don't think you can find a major tire manufacturer that will willfully step over the line and recommend LT tires for replacement tires where there is no such recommendation from the trailer manufacturer. (Unless the LT tire is rated by it's manufacturer as a trailer tire). A letter or phone call from a manufacturer's technical expert does not carry any weight without the actual tire manufacturers official recommendation (s).
FastEagle