I've never had trouble out of Goodyear Marathons...I hear that the Carlisle tires are fine too...

It sounds like you're keeping them inflated properly and checked regularly, so that's good. Do you have to add air to them often? If so, you could have a slow leak in the wheel or around the valve stem causing the air pressure to drop slowly after you inflate and are bouncing down the highway...then an under-inflated tire builds up excess heat and can cause the failure. Even the cheapest made tires on the road shouldn't have a tread seperation issue...that's a pretty big defect...so I generally side towards air pressure problems caused by leaky wheels and valve stems, or road hazards that you might not have seen. Of course, it could be a defect too, but chances are less likely.

Let a good tire man look at your tire and they can look for things that point them in the right direction, but there is no reason not to trust a Goodyear trailer tire.

As for Nitrogen fill, it's becoming more popular. If your wheel/tire is well-sealed, you shouldn't lose air pressure and it won't fluctuate with temperature. You still need to check it regularly, but it should be stable. The Nitrogen molecules are bigger too, so it is less apt to escape from tiny cracks that might be present in your wheel/tire/valve, etc. It shouldn't cost you that much to Nitro-fill a boat trailer tire though. I know what Nitrogen costs, because my store has a mobile nitrogen filling station, and it can be done pretty inexpensively.