All my trailers sit in the tractor shed or the trailer barn out of the sun light. The only time their in the sun is when used. I'm not a wealthy person so my trucks and trailers have to last many years.
I've had my current 5er for 19 years and use Tire Care (all cars and trucks) about 2-3 times a year. I use LT tires on all my trailers and run them for 50k-55k miles and 7-8 years before selling them and replace with same.
I've tried 303 on my truck tires and found it doesn't stay on near as long as Tire Care.