Sorry for the late response, but I just found this thread.
Funny thing you should ask about a leaky roof vent. I went to an Extended Learning Class on RV Care & Maintenance last week, put on by the Community College. The Instructor was a Service Manager for a large local RV Dealership.
As for a sealer, he recommended Dicor as the only sealant to be used on a rubber roof. NEVER use any petroleum based products on a rubber roof.
As for the vent being cracked he said that you could use Dicor as a temp fix. He strongly recommended replacing the cracked part because if you just seal it, it creates a dam around the vent and could continue the leak. He said that the part was not expensive($15-$20) and you could replace it yourself. Remove 4 screws on inside and the screws on the roof and remove cracked part. On the new part place some tape seal all the way around on the bottom of the part. Reinstall the part and screws. Cover the screws with Dicor and around the edge of the part. Reinstall the inside screws and you're done.