From the looks of things, this is the caulk that is in contact with the rubber roof. If I remember correctly (and I'm probably not!), use Dawn dishwashing soap to clean the area around the caulking & roof, rinse it good, and let it completely dry. Then use Dicor self leveling caulk. Never use silicone or any other type of caulk on the rubber roof, as it will damage the rubber roofing. The new Dicor self leveling caulk can be applied over the old caulk, but it must be cleaned good so it will stick to the old caulk, and rubber roof. It can be bought at any RV store. If I'm wrong about prepping the roof prior to applying the new caulk, someone will soon correct me!