My understanding of the issue from back in my trailering days was that you Don't use Radial Ply tires. I see most of the previous posts dancing around the issue with "stiffer sides" , not "passenger tires", etc... That's the biggest difference between radials and non-radials. The trailers can't handle the squirming/side flexing that radials are Designed To Give You. When I went to a trailer place it was simple - Don't Buy Radials.
Wickford/Narragansett Bay RI