On our 15k 5ver I upgraded the tires to g-rated Goodyear g-614. They are 14 ply but with nearly 8k miles, they still look new. If I had to guess I'd say maybe 80-90% remaining. Honestly can't tell any wear yet. They are expensive but worth it in my opinion.
Brett
Time is the enemy. 6 years plus they will be a ticking time bomb no matter how much tread is left.
Chris
What kind of milage are y'all getting from your tires? I've got 5k miles on this last set on 10klb tag consistently hauling 6klb. I will be luck to get another 1,000 mi before they need replaced.