Obviously, RV and boat trailer manufacturers are simply willing to take the cheapest way out. On the other hand, many people with tire problems have them under inflated and are not always careful going over curbs and railroad tracks.
Have you tried to find high quality ST tires locally? I don't think that there are enough good ST tires in North America to service the industry. The manufacturers have to go to China because not enough North American tires have been built. Great tires for heavier trailers are Maxxis M8008's, Goodyear G614's, Bridgestone R250's and Michelin XPS Ribs. (Some are LT 10 ply truck tires.)
I'm switching over my new trailer tires to Bridgestone Duravis R250's, and there's a $70 rebate until 8/6/2014.