Crewdog,
As others have mentioned, mileage on trailer tires is only one factor in judging when to replace the tires. In fact, I would suggest the following are the key factors, with generally the most significant factors closer to the top of the list:
1. Age of tires since manufacture, not when installed on the trailer
2. Signs of side wall cracking
3. Whether the tires have ever been run under-inflated
4. Sitting time un-used on the trailer
5. Multi-axle scrubbing that has occurred
6. Tread life remaining
7. Miles traveled on the trailer
I have an older uncle who still farms. On his farm equipment that is never used on a road/highway, he runs tires until they almost fall apart, regardless of age or tread depth. He knows that at slow speeds in a field, a tire failure is not catastrophic.
However, he also has had 3 different 5th-wheel RVs over the years. On those 5th-wheel RVs, he buys new tires every 2-3 years, regardless of tread depth or miles used, since he and his RV buddies have learned the hard way that tire failure on an RV trailer at 60-65 mph can be harrowing, along with a major inconvenience to deal with on a trip. Same is true with a boat on a trailer.
My experience with 2 different diesel-pusher motorhomes over 11 years and about 96,000 miles traveled is to always replace tires every 5 years or less, even with low mileage and/or good tread depth. These motorhomes use truck size tires, so after 5 years and many tens of thousands of miles the tread depth looks almost as if the tires are new. Most of my motorhome friends view tires the same way. A tread separation or blow-out on a motorhome pulling a toad is not something you want to experience. I suspect the same is true when pulling a trailer with a valuable Ranger Tug on it.
However, on my automobile tires, I've usually replaced tires after the tread depth approaches the limit, rather than considering mileage, years, etc. Auto tires seem to use up the tread depth before other factors become too important.
