Yes the "R" in a tire's size description (ST225/75R16E) identifies it as a radial design.
All tires have a short list of their basic building materials listed on their sidewalls. That includes sidewall materials.
Here is a picture I took of a tire's materials description.
http://www.irv2.com/photopost/showfull.php?photo=27063
Most steel cased tires used for RV trailer service right now are 16". However, some 15" ones will be on the market soon.
Most trailer tires have a rim width range that could look like this; 6"-7" with the measuring rim being 6.5". To date all of the 16" steel cased tires have a single "approved" rim size of 6.5". Some of the newer brands are so serious about it they are putting the required rim width on the tire's sidewall. Here is a picture of one I took at a RV show earlier this year.
http://www.irv2.com/photopost/showfull.php?photo=29709
No savvy tire installer would ever mount a tire marked for a 6.5" wide rim on anything other than the required 6.5" wide rim.
A tire's strength is in its casing and the amount of tire inflation it can sustain. Belts, overlays, sidewall inserts and treads are all durability items, not to be confused with load strength.