Gulf of America

US citizens. I do real estate signings and the Patriot Act requires proof that you are a "United States citizen"

Yes, it's all kind of splitting hairs. It's easier to say "I am an American," and overseas you are like to be asked if you are an American.
Fair but I've certainly heard American citizen as well

But just conversationally?

Technically you can say people from Germany are German citizens but everyone just calls them Germans

Legal and official paperwork is one thing but informally I don't know how often US citizens is used