As stated above, the water is either coming into the camper where the roof vent is attached, or it is coming in through/around the vent lid itself. Possibly from snow piled up around the vent.
If you have had the trailer a year, there is a good chance the Dicor sealant around the flange of the vent has shrunk and/or cracked, allowing water to enter. It doesn't take much of a void to allow water in.
Clean the area around the vent. Inspect with a strong light. You will probably have to re-caulk with self leveling sealant suitable for rubber roofing.
I've never tried to caulk anything in really cold conditions. You may want to research that aspect. Good luck.
