Yep,Sunlight will eat a tire up like crazy. I bought a new trailer in 2005, and the sun cooked them pretty good in about 10 years. I put a tube in the one that had the most sunlight, because the "checking" caused the air to leak out of the sidewalls. This year I bought a new set of tires and rims for it, since I was going to use it again on the road.
I have a set of tires that see no sunlight, which were bought well before 05' on my boat trailer that is stored in my basement. These tires look just as good as they did when bought new!