Ron and Heather,
If you have cracks in your sidewalls you need to replace them. That's usually a sign of UV damage. I would always recommend covering your tires when the trailer is parked outside. Even though my dad always told me that and he did it, it took be ruining a set of tires on my last trailer in only two years leaving it outside. All of a sudden it made sense for me to buy four covers at about $22 a pair.
mac