I rotate my tires about every 3000 miles. On the left side I just move the front tire to the rear and move the other 2 forward. On the left I put the spare tire in the mix and move the front tire to the spare position. At this time I inspect my suspension and frame for damage.
I have never had tires fail due to "dry rot". I have always worn them out. If your tires are falling apart you are doing something wrong. Do you keep them covered and out of the sun? Do you store your trailer on dirt? Do you wash them off regularly?