A lot of people have put on a lot of miles with no problems from the Chinese tires. By the way, it's not just Goodyear Marathon that's suspect. It's all tires made in China.
We have had numerous blowouts with Marathon, Mission, and Freestar tires, all made in China. It's only when we switched to a quality US-made tire that our tire problems went away.
I suspect part of the problem is the tires have ratings that exceed their actual capabilities. Over-rated, in other words.
If your rig is light enough that you don't exceed somewhere between 50% and 75% of the weight rating stamped on the sidewall AND you are a weekend camper and occasional vacationer, then I'd probably stay with the Marathons until they prove to me they are unworthy. If your rig runs heavier than that or you put on significantly more miles than that, then I'd probably swap them out.
It never hurts to be safe and go to a better tire. But if the cost is a concern, as it is for many of us, then I'd evaluate my situation and decide how much risk I'm willing to take.
Safety is always a concern but I do want to say that with every single blowout we've had, blowout and ordinary flat tire, there were only three ways we knew we had a problem back there.
1) We heard the pop of the blowout (sometimes we did, sometimes we didn't)
2) Someone pulled alongside and signaled us. We never felt a thing.
3) We happened to see a bubble on the side of the tire while in an rv park.
I'm not suggesting anyone cause themselves any risk. I'm just saying there are some situations where the Marathons will probably be just fine. Our situation is not one of those, however.