Hi, I bought a set of four new Carlisles for my trailer just before my cross country trip, seven years ago. I got my first flat tire ever, on my trailer, about two weeks ago. I think they are good tires, but this time, I bought five Goodyear Endurance tires. Mostly because of good reviews and made in USA.
On my Alaska trip, I chose not to replace my tires because of the road conditions, I didn't want to tear up new tires. I, at this time, had two seven year old Marathons on my trailer and they both had shown signs of seperation at different times. I bought one Carlisle and one Marathon because when you are in the middle of no-where, you buy what they have.
On my tow vehicle, 2000 Lincoln Navigator, we stopped in Montana for new tires. This was on our way back home in Southern California from Alaska. I had good tires when we left our home, but the gravel roads ate them up.
I think the Carlisles are good tires. Goodyear Endurance, in our size, only comes in Load "E" so I run them at 65 lbs. pressure, same as I did my load "D" tires.
Answer to your questions. I would keep those tires on your trailer for his trip. I don't believe that you will ever wear out trailer tires before you time them out. For me the magic number is seven years, but that is pushing it.