jamas, do you have any photos of the cracking? generally if i only see cracking around the shoulder of the tire, i dont recommend new tires unless the cracks are large enough to fit the edge of a coin into

when you see cracking in between the treads of the tires or right around the rim, thats when i start to get concerned

i think UV must have at least a small part of it, ive never seen a spare tire mounted under a vehicle with dry rot on it