In the last tire picture, you can just see the DOT info in the upper right corner.
It appears to read 1808 or 1809, which would translate to either an early 2008 or 2009 built tire. That means 4-5 year old tires
Add to that the fact that the rig has been sitting for a minimum of 1 year, and I'm not at all surprised to see dry rot in these tires
When I bought my truck in AZ, it had sat on a dealers lot for almost 6 months in the AZ sun. Within a year, I was seeing the effects of dry rot. Michelin replaced all 5 tires on a pro-rated basis
JMHO