There's a reason to get rid of six-year-old tires.

I ran Michelin XPS Ribs on a previous 5th wheel with 5200 lb axles for 6.5 years and sold them for 200 bucks on CL with around 40K miles on them and 65% tread left. Guy that bought them was tickled pink to put them on an old Ford that he used for garbage dump runs.