The sealant is not even required, as the hole into which the screws go to hold the Top portion of the vent cover do not go all the way through the base.
The sealant will, however delay the rusting of the mounting screws, which look like a long Deck Screw and are normally made of poor quality metal.
The best option is to use a stainless steel screw to prevent the rust problem.
Also, there are (or at least were when I was in the business) two major roof vents and they look somewhat alike, Norcold and Dometic. They are not interchangeable unless you also change the base.
CAMCO also came out with a "universal" Fridge Roof Vent replacement top that will fit either the Norcold or Dometic, and it is less expensive.
Funny story, as an aside. When CAMCO first came out with the their "universal" top, I had a customer buy one and return it stating that the holes did not line up. He had a Dometic Fridge. When I called CAMCO, their first question was "Is it a Winnebago?", which it was. Turns out Winnebago has their own plastics plant (or used to) and molds a lot of their items that "look" similar to other OEM products, but are not exact duplicates.