The possessive form of "campus" is campus's.
This applies to both singular and plural forms of the word. For example, you would say "the campus's library" or "the campuses's libraries."
However, in some cases, you can also use the apostrophe without the "s" if the word is already plural. For example, you could say "the campuses' libraries."
Ultimately, both forms are grammatically correct. However, "campus's" is generally considered the more formal and preferred option.
