Self-leveling lap sealant is excellent to use on relatively flat surfaces as RV Wizard says.
Another good product with higher viscosity for use on curved or vertical surfaces or where you need to make a bead such as around clearance lights is
Pro Flex RV Sealant. It's a little tacky and difficult to get a real smooth bead, but it adheres better for me than silicone and a perfect bead isn't really necessary way up on the roof. ProFlex is paintable and adheres well even if the joint is slightly damp or not perfectly clean. It is available in white, clear, or almond. I have used ProFlex on bare fiberglass, painted fiberglass, vinyl, ABS plastic, and metal.
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