I would go with Aerospace 303 Protectant. It`s relatively inexpensive and easy to find over-the-counter at most boating or RV supply places. Just make sure the plastic/vinyl is clean before you apply it. Aerospace even makes a vinyl cleaner for that purpose. I use Meg`s Detailer Line D101 All-Purpose Cleaner (the green stuff) as a plastic pre-cleaner.
One thing about 303 is that, depending on where you live and how much sun-and-heat the plastic will be exposed to, it may require a more frequent re-application of the protectant than some of the coatings mentioned above to offer the UV protection, as it does gas-off (evaporate) in the heat over time. I would guess an every-other-week application may be in order if you use the boat a lot. One other suggestion is to use a cotton-cloth (like a flour sack cloth) to wipe off and buff excess protectant applied. Using a microfiber cloth for wipe-offs tends to be too absorbent and will remove too much of the just applied protectant, causing blotches and streaks on the plastic/vinyl. You can let the protectant sit for a 3-5 minutes, but I do not like to let it sit too long, or evaporate on its own in the heat, again causing blotches or streaks.
Edit: I see the OP wants some thing for EXTERIOR trim. My suggestions are more for interior trim. I`ve used Meg`s consumer-line Ultimate Black trim Restorer for exterior soft plastic/vinyl trim, but that`s an automotive-based application.
Meguair`s makes a marine-specific vinyl and rubber treatment M57 that may apply to meet your specific requirements.
Lastly, check out McKees 37 RV product line. While not marine specific, I am sure that some of their rubber and plastic care products may also cross-over for your application.
