Our rig had the GY670s on it when we bought it in 2010. The date code said they were 2001 manufacture! After driving the 30 miles home from the seller's home, we took a 2-night trip to a local SP, just to get a feel for the living accommodations. It was about 70 miles each way.
My feel for the GY tires was not favorable. I found them to be noisy and rough riding. I decided to go all new (except for the spare wheel, which I don't think has ever been used.) Since we're in Les Schwab country and have been their customers for our car tires for over 25 years, we went to our in-town store and bought their top-of-the-line Toyos. The store manager found a set of six that had a date code less than 6 weeks before we bought them.
We're very pleased with the choice. They ride quietly, seem to not be upset by being stationary for long periods and are comfortable. We've only put 5500 miles on them in four years, which I think is fairly typical for recreational users.