IS A 10 PLY BEST FOR???

From everything I read from the tire manufacturers is ST is not a marketing classification as some seem to believe. Since the funcion or trailer tires is just to follow the tread patterns are different. Since they squirm instead of turn and since side winds affect trailers much differently that it does a truck, the sidewalks are considerably stiffer. This also helps to eliminate sway. Maybe take a look at Goodyear Marathons. My trailer is 10k and my third set of Carlisle Radial Trails have been good. I suggest not going anything less than E rated and maybe even consider G.
Happy trails!