All tire manufacturers recommend driving the vehicle occasionally to help bring the emollients to the surface. These emollients keep the rubber pliable. When tires sit for months on end the outer surface dries out.
The surface becomes brittle and will crack at the stress points. Given that the sidewall has the most stress, and is the weakest part of the tire it would follow that it's the most likely place to crack.
Here's a link the the Michelin RV Tire Guide:
https://www.michelinb2b.com/wps/b2bcontent/PDF/RV_Tires_Brochure.pdf
Pages 7 and 8 cover storage and excersizing the tires for long life.