Tires that get used move and flex which helps to get some of the rubber UV protection in tact. Tires that sit around for long periods need UV protection. Wheel covers are a great idea.
303 protectant works. I have used it on canoes and rafts for years.
I had a nice horse trailer years ago with two axles. I did not worry about UV protection much because I used the trailer pretty often. One day I hears a loud bang and one of the tires exploded. I shutter to think what might have happened with critters in the trailer at highway speed.