I'd be willing to bet there are no tire manufacturers recommending LT tires over ST tires for trailer applications. Legally speaking, makes no sense for them to do so.
Out of curiosity, which fifth wheel manufacturers are supplying their product with LT tires? Seems like that would be a legal issue in the case of a blowout/wreck "X product was delivered from the factory with non trailer tires and caused X incident". Not saying that has happened, just trying to make sense of it.
Just throwing that out there, as it seems odd to me.