I personally suspect with Marathons, that in some cases overloading is the cause. I think it is pretty easy to overload them, particularly if a weight distributing hitch is used (and a weight distributing hitch usually is a good idea, so it should be in most cases!). I also think TM used some of the defective valve stems that were recalled a few years ago.
I got 20,000 miles out of my Marathons. I replaced them this year just because of experience on this forum with them and that I didn't like running so close to speed ratings and weight ratings. They were 3 years old when I replaced them. But they never gave me problems. I did balance them (which is good for the wheel bearings even if it isn't necessary for the tires), checked pressure religiously, greased wheel bearings yearly, verified brake adjustment every few thousand miles, and never left it sitting very long in one spot without moving it. I think those things help.
I also overinflated them by 10 lbs, as I felt that would help them hold up better so close to their limits - others may disagree, and I'm glad I have a tire I don't need to do that with now (note that the tread wear pattern on the original tires - I ended up with 1/8" or so of tread - indicated that this was too much inflation). I'm also glad I have a tire that can go 70 MPH occasionally (to avoid elephant races - I normally tow at 65 MPH or the speed limit, whichever is lower, but occasionally want to pass a semi going 64.5 MPH!).
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