No matter how much we bloviate the bottom line is LT tires have a much lower reported failure rate on heavy trailers (Load Range E) than ST Tires.
ST tires are used by the RV manufacturers not because they are the best tires for the application but because they cost less. Like most things in the RV the components come from the lowest cost bidders.
ST tire manufacturers spec ST tires to be good for 5,000 to 10,000 miles and/or need to be replaced in 3 to 5 years according to the Tire Rack site.
Would you put that kind of tire on your tow vehicle? I've got 70,000 miles plus on my Michelin LTX M/S on my TV with 15 to 25% tread left without a single failure. Meanwhile have had two sets of ST tires fail on the 5th wheel each set with less than 10,000 miles. All three sets of tires were on the same roads at the same time. No, the 5th wheel tires were not overloaded. No, the 5th wheel tires were not under inflated. No, I did not travel faster than 65 mph. What is wrong with this picture?