I keep my trailer under cover when not in use, but I wipe all plastic parts with 303 UV Aerospace Protectant every 3 months or so. I have used it for years on all of my trailers, past and present. It seems to keep the platic parts from yellowing and/or UV induced deterioration.
2015 F150 2.7 EcoBoost (Big Spot)
2015 5.0TA (Little Elsie)
"What a long, strange trip it's been....."