I run 8 ply radial trailer tires. Mine are like Mac Daddy's no name from gateway tire. Had them 3 years and barley show any wear. I know I don't run the miles you do but these have impressed me against all the trailer tires I have gone through the years. Goodyear was supposed to have come out with a new trailer tire to replace the Marathon that was supposedly better. Don't know that to be the fact. But radials are only way to go and 8 ply minimum. 10 if you can find them. And run 50 psi at all times. Tire pressure is your biggest killer of tires especially in this weather.
Proud Member of Team Geezer
Charlie Weaver USN/ENC 1965-1979