Trailer tires typically have stiffer sidewalls to help prevent sway in crosswinds. That is why bias ply tires stayed so long on trailers because of their inherant sidewall stiffness.
Most trailer tires now are radials, but they have extra belting to increase sidewall stiffness.
Car tires will work just fine on some trailer applications, and last a long time because of their comparitively light loading. But with trailers that are suceptible to crosswinds, such as campers, specific trailer tires are highly recommended.
Mike