Rhode Islander vs American

Rhode Islander and American both are nouns.

Word	Noun	Adjective	Verb	Adverb
Rhode Islander	Yes	No	No	No
American	Yes	Yes	No	No

As nouns, American is a hypernym of Rhode Islander; that is, American is a word with a broader meaning than Rhode Islander:

Rhode Islander (noun)	American (noun)
a native or resident of Rhode Island	the English language as used in the United States
	a native or inhabitant of the United States
	a native or inhabitant of a North American or Central American or South American country

Rhode Islander (adjective)	American (adjective)
	of or relating to or characteristic of the continents and islands of the Americas
	of or relating to the United States of America or its people or language or culture