There are probably two different seal materials. One will be under the metal flange of the vent and it will be the gummy tape type that others have mentioned. The other is probably just a water seal bead around the top and side edges, mostly cosmetic but aids in keeping water from penetrating behind the flange. Any decent brand of RV caulk, e.g. Geocel Proflex RV, should handle that. The flange doesn't get all that hot or the sidewalls and paint would be damaged.