I recently purchased a new to me 2011 travel trailer with the original tires where the tread looked like new. One tire had very minor cracks in the side wall near the bead and I've replaced them, keeping the spare. I've heard too many stories of people losing a tire while far from home and can't easily find a replacement. A few years ago I was ignoring the age if the tire looked good and lost a large chunk of the tread on tires I had bought used, ten years previously. I felt the bumping, got out and looked at all tires, truck and trailer. All looked good so I continued on my way, thinking I had a spark plug problem, jerking the truck. After another few miles at 60 MPH, I looked again to see the missing chunk but still holding air. Don't want to repeat that experience. My plan now is to run the new tires for about five years and then find a buyer for them to subsidize the purchase of new ones.