Yes, bedrooms is the plural form of bedroom.
To pluralize compound possessive nouns, add an apostrophe and an "s" to the last word of the compound noun. For example, "my brothers-in-law's car" is the possessive form of the compound noun "brothers-in-law" in plural form.
bedrooms
Coaster's is singular possessive. The plural is coasters, the plural possessive is coasters'.
The correct form is "its" for the possessive form in the plural. "Its" is used for both the singular and plural possessive forms, without an apostrophe.
The plural is briefs. The plural possessive is briefs'.
The plural is lads. The plural possessive is lads'.
The plural is trollies. The plural possessive is trollies'.
The plural is stations. The plural possessive is stations'.
The plural is relatives. The plural possessive is relatives'.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive form is heart's; the plural possessive form is hearts'.