I change mine every 5 years as well. I make sure I buy fresh stock when I buy the new ones . Nothing wrong with them when I change them and heaps of tread, tread almost like new actually.
I replaced mine for the second time last year I bought Bridgestone 205x70x 15" 8 ply radials, made in Japan. I will not buy any tyres made in China.
My trailer built in USA came with crap trailer tyres, I got rid of them soon after the trailer arrived in AU.
I would also suggest you cover your tyres when parked in the sun, especially in hot climate like Fl.