Well, I just looked in Tire Rack under Light
Truck and they have a 'trailer service' tire in 13" called the Goodyear Marathon...
I recall looking for LTs in 15" for my previous truck and they were not common and cost a bit more than I cared to pay.
Here's another Discount Tire link I found:
Why Use An "ST" Tire
* "ST" tires feature materials and construction to meet the higher load requirements and demands of trailering.
* The polyester cords are bigger than they would be for a comparable "P" or "LT" tire.
* The steel cords have a larger diameter and greater tensile strength to meet the additional load requirements.
* "ST" tire rubber compounds contain more chemicals to resist weather and ozone cracking.
To compound the issue, there are apparently LT tires made specifically for trailer use in larger sizes that should not be used on actual trucks because they are not intended for cornering...