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....
The tires on trailers/5th wheels do not wear the same as automotive tires. It is very possible that the tire can look practically new after many years use, but it is recommended that they be replaced every 6-7 yrs. or could be subject to failure.