BOAT TRAILER TIRES

I have a 22 ft. bay boat on an aluminum trailer with tandem axles. It came with the Goodyear marathon tires .
4 tires have blown out with me in the last year. one had less than 500 miles on it. The Goodyear dealer here has given me every excuse in the book, but hasn't done anything to try and fix the problem. I had one of the blow outs with the trailer setting in my yard.