Almost EVERY tire store will have some 'expert' that knows more than anyone who actually uses tires. Problem is that a whole lot of people don't know anything about what kind of tires they should have anyway, so this 'expert' gets to tell them what they need and they buy it.
ST tires are for trailers and they all are only rated for 65 mph at most and very close to the load rating. LT tires are ALWAYS rated for higher speeds and have a much higher load rating 'buffer', since their supposed to be used on trucks, not trailers. LTs also have more flexible sidewalls than STs. The sidewall thing was what convinced me to put them on my Monty. I've a narrow street and narrow driveway that I've got to back this thing into - you'd not believe the bending these tires have to do to get into it! After 6k miles they still are working well.