Eventually happens to all TT's, but I guess the higher temps and stronger sunlight in the southwestern states doesn't help.
I use 303 Aerospace Protectant on my plastic trim and a Filon sidewalls, great UV protection.
My plastic entry door window trim and outside fan vent cover yellowed (prior to using the 303 stuff) and I used a Rustoleum plastic spray paint
http://www.rustoleum.com/cbgproduct.asp?pid=118 that worked great.
Don't know if warranty will cover something like that, but you can try (sooner better than later).
Bob
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2016 GM 2500HD 6.0L/4.10
2018 Jay Flight 24RBS
2002 GM 2500HD 6.0L/4:10 (retired)
2005 Jayco Eagle 278FBS (retired)
1999 Jayco Eagle 246FB (retired)
Reese HP Dual Cam (Strait-Line)