There is a fellow I know slightly who operates a long established RV business in Ontario. He also writes a column on towing issues in a Canadian RV magazine
www.rvlifemag.com
I wrote to him, asking his opinion of the ST tires that are now commonly supplied on most trailers. (He sells Airstreams, Sunnybrooks and HitchHikers). Here is his response. I think that the first paragraph says a whole lot! Also, note that he says Goodyear make a tire with a dual LT/ST designation.
Hi John
Many of the trailers we sell we change the tires to LT tires from the
ST.
It is pretty much as you suspected. As near as I can tell ST just means
not good enough to use on a car. We do not have a lot of experience with
the 16" Marathon's as yet but I suspect they will be about the same
quality as the 15's. Which is acceptable but not great.
I would stay with the LT tires when you replace your tires.
The only issue we ever had with LT tires was Goodyear once made a tire
where the sidewall plies wrapped around the steel tread plies. The
turning action of the trailer where the two tires fight against each
other caused the steel belts to saw through the sidewall plies and
Goodyear had to recall all the tires used on RV's.
Goodyear makes LT tires with a small st designation on them.
I have never had a problem with Michelin, Bridgestone or Yokohama
tires.
I would only recommend the "G" tires where you absolutely need them to
carry weight as they are pretty stiff riding on lighter trailers.
It would be a good idea to weigh your trailers axles sometimes just to
see what weight you are carrying on the tires.
Thanks for writing.
Andy