Andrew, we'll give you the benefit of doubt since you're fairly new. I'm going to ASSUME when you say silicone you mean it in a generic term. Silicone is the Devil's Bane when it comes to
fiberglass trailers. It's a temporary solution that can become a permanent problem.
I'd use Geocel ProflexRV sealer although there are other good caulks. If you don't feel confident that you'd get a smooth even bead. Stripe the upper section with blue painters tape and squeeze out a small bead. Others may have additional ideas.