So...what was so different about the wear pattern in the spare. If you replaced your tires two years ago with G614's, did you replace the spare too? What is the date on the spare? Was it used before? For how many miles? Do you know the history of the tires? Did you own the trailer from New? And the original tires?
Kinda hard to give you definitive advice without accurate history of the situation.
Disregarding that. I don't like Goodyear tires. Marathons, G614's, whatever. They are all junk. Probably why the 2 year old G614's blew out. And GY is overpriced too. The good news is that Michelins aren't any better. But there are a lot of better tires out there.