OP when you say vent and vent cover, by cover are you talking about an added raised up extra section that supposedly allows the vent to be open "rain or shine" as they advertise. If that's the cover you are referring, it is not sealed to the roof typically, just clips on, and depending on how much rain and wind, and wind direction, rain can easily get in if the vent is open.
2021 Thor Omni XG32
2020 Ram 1500 Laramie (toad to be)
2019 Harley Davidson FLHTC