Our TrailManor 3124KB came from the factory with Goodyear Marathon st225r75 15 load range D tires.  The weight on the wheels is about 3200 lbs, single axle (65 psi, 24.6 square inches of road contact per tire).  At 9500 miles we suffered a total sidewall failure - the sidewall separated from the tread.  We were lucky - it happened in a city and caused little collateral damage.  We went to a Goodyear factory store (some stores are independent licensees) and they provided a 90% replacement.  I have asked if they'll offer a similar deal to the ticking time bomb on the other side of the trailer; I'm awaiting their answer.
The tires are about 6 years old and the trailer is stored outside.  The tire that blew is on the north side of the trailer as it's stored, so I don't think UV can be the culprit.  (The one on the south side is also out of the sun due to the trailer cover and the tall vehicle to the south.)  Weight is on the tires all the time.
I was impressed with Goodyear - they certainly could have shirked their duty, but they stood behind the product.
I've read on other forums Marathon's produced 5-7 years ago are highly suspect, but that process and/or design changes make newer ones more reliable.

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