I think there "might" be some confusion in "types of plys" being discussed. There are "body plys" (usually polyester, Dacron or nylon) and "tread plys" (usually steel in radial tires) which are covered by a "nylon ply" which helps prevent the steel belts from shifting and separating.
I don't know of any LR C, D, or E (6, 8 or 10 ply) "all steel ply" tire that's marketed for ST use. All have polyester "sidewall plys". Some Goodyear truck tires have started using Kevlar as an additional ply to help strengthen the sidewall for off-road protection, but even they don't have "steel sidewall belts".
ADDED: This is a quote from the Tire Rack website.
http://www.tirerack.com/tires/tires....=Towmax+STR+II In it, they describe the STRII composition this way:
"The tire's internal structure features twin steel belts (Load Range D & Load Range E-sized tires are reinforced with a nylon cap ply that further stabilizes the tread to increase durability). The Towmax STR II's rounded profile and polyester cord body are designed to help smooth out the trailer's ride." As you can see, the "sidewall plys" are polyester.