Totally by quiescence I have two sets of Goodyear Marathon trailer tires.
Set #1 is original on our 2002 25" Baja trailer. They still look like new. They only go 10 miles in the spring to put the boat in the water and another 10 in the fall to take it out.
Set #2 is on a 28' car/ATV trailer we bought in 2005 that gets used about 500 miles a year.
Not a lot of miles on either set, but I believe the reason they have no cracks, check marks or issues is because both of these trailers are stored inside all year long. Sun is what kills trailer tires. IMO, it's worth buying covers for your tires when they are not in use.