I'm not an expert but have avoided the problem by driving the van every 2 or 3 weeks and storing it under a roof so the sun doesn't get to the tires. Tires have an additive to protect against dry rot that is released when the tire heats up with use. Also I found this:
For the record, Michelin recommends that:
•If the cracks are less than 1/32" deep, the tire is O.K. to run.
•Between 1/32" and 2/32", the tire is suspect and should be examined by your Michelin tire dealer.
•If the cracks are over 2/32", the tire should be replaced immediately.