I'm a retired tire shop manager and I can tell you none of the trailer tires are made in the USA. Goodyear just started making some within the last year and it is not the Goodyear Marathon. I cant remember the name but they are made by Goodyear. All other trailer tires are made overseas. The tires on your trailer maybe Bias If they have a D in the sizing it is bias, if it has a R in the sizing it is a Radial. Also trailer tires should be run at or close to maximum inflation which is on the tire. Another thing if possible try and upgrade to a 8ply tire. Most of them will come with 6ply. That will depend on the size tire you have if they offer it in 8ply. the cost should only be a few dollars more for the 8ply, I mean maybe 3 to 5 dollars a tire more.
Hope this helps you!
Mark