I have a fifth wheel with Goodyear G614 G rated tires. They still have very good tread and still look good but they were made in the 19th week of 2014. I read that you can go between 6 to ten years....
Just a word of warning. Not always the case, but beware of tires made in China. I have been advised by many sources that tires from this country tend to blow. In any tire, be sure your air pressure is correct and check tires at rest stops with your foot to be sure they have a degree of air in them and your hand to be sure they are not over heating.