People are so quick to blame made in Ch*na tires or the brand which is so wrong. Many, many owners have had excellent success with Marathon tires and with other tires made offshore. We have Marathons and expect long life from them. It has a LOT to do with how you treat them.
500 miles is waaaay to short so something has to be wrong, especially if the same wheel.
Unless it's a new unit and has not had the bearing removed and repacked, it could also be a really loose wheel bearing. I helped a fellow camper replace a flat tire in a CG a few days ago. He bought the TT used and the PO had removed and repacked the bearings which he did not tighten properly and the new owner said he found it had up to around 1/4" of play when he checked it once. The tire was severely worn on the inside and I would have guessed a bent axle... He thought that wheel was the only one with a loose bearing but since we got both wheels off the ground, I noticed the the bearings on the other wheel were loose, but maybe 1/16" - 1/8" and he was a bit shocked to find out. He said he would check them all when he got home.
I guess the moral is that when buying a used TT if you know nothing about the history, at least check the bearings for play, especially the older it is and even better, remove, inspect, repack and re-install.