mus' be the heat making grumpy people even more grumpy.
michelin Q&A
*What is the expected service life of tires?
While most tires will need replacement before they achieve 10 years, it is recommended that any tires in service 10 years or more from the date of manufacture, including spare tires, be replaced with new tires as a simple precaution even if such tires appear serviceable and even if they have not reached the legal wear limit.*
For those that aren't familiar with the XPS Ribs...their a commercial grade all steel ply carcass tire that Michelin recommends for commercial trailers. They work great on rv trailers as reports we see from time to time mention running the XPS in excess of 48k-50k miles on their trailers.
I wouldn't attempt using a poly carcass tire in a trailer position that many years.