Most of the times ST tires get a bad name due to a couple of issues. Age, air pressure and overloading are the main reasons people have trouble with them. If you keep a good check on the air pressure, change them every six years, and never approach the load rating max, most people don't have issues with them.
However, and this is "not recommended" by tire manufacturers... I run LT tires made for a pickup truck on my trailer. In fact, I run the exact same tire on my trailer as I do my dually, so I only need to carry a single spare. (Bolt pattern is the same, by design). Most tire manufacturers don't want you to run car or truck tires on a trailer. Something about linear direction or pulling only... instead of being a drive tire or some mess like that.
If you are having issues, take a close look at the tare weight of your truck, and what the load is your putting on it. (Gross weight). Make sure it isn't approaching max on the tires. (Most trailer companies (especially RV trailers) like to cheap out on them, giving the new owner problems. Spend the extra money that you are spending on ST tires and buy a higher load rating tire. Calculate the highest load you will put on your trailer, and add as much of a buffer as you can find!
If you can't find better tires (15's are much for load rating).. look into adding a larger sized tire and rim. 16 inch rims offer a lot better tire assortment and higher load range tires. You have to make sure that tire clearance between each other and also under the fender (especially when loaded) but once you do, you will not have nearly the tire issues you are currently having.
However, as bad as ST tires are, going up in load range and or size is never cheap...
HTH,
David from jax