Between year four and five on these Kumho 857's ("commercial LT", marketed as "trailer tires" in north america ) on this travel trailer I'm in right now. No cracking, and still have more than 1/2 their original tread depth. Approx 40K to 45K miles.
I don't cover any of my trailer tires to keep sun off them. I've got too danged many trailers that are being used at odd times to be messing around with tire covers. I'm sure covering them is a good thing, but I'm just not gonna do it.
Have run some Michelin tires on a few of our cars over the years, including on my grand-daughter's Ford right now. No cracking. Have also run a few sets of Michelins on street bikes in years past and did not have any cracking issues.
Another side note regarding these 857's. They are running at about 45% of their rated load in pounds. But still, I run them at sidewall max ( 65psi ). Some folks would argue I am running over inflated, and therefore should be seeing a classic wear pattern indicating over-inflation. However, they are wearing exactly evenly all across the tread face, and all four are wearing the same amount. Again this is after 45K miles of use.
Your mileage, inflation and cracking, may vary :)