In English, American means from the US, and there's no word to refer to an inhabitants of the Americas (both continents combined). You can say North American or South American if you want, though. Since those are continents.
You won't find "USAnian" in any authoritative published dictionary, not even the OED:
https://www.oed.com/search/dictionary/?scope=Entries&q=USAni...
https://www.urbandictionary.com/define.php?term=usanians
And anyway, official dictionaries are largely historical records, not authoritative sources for living languages. Words mean what people who use them intend them to mean.
It's not.
Anyone can put anything in Urban Dictionary, c'mon. Nobody said no one has uttered the term before.
If something is "often used", it winds up in dictionaries, with a lag of only a few years.
https://en.m.wiktionary.org/wiki/Usanian
I’ve heard or read the term at least once or twice along the way, I’ve even muttered it myself.
It might not ever rise to a common enough usage that the big dictionaries list it, or maybe it will.
I probably wouldn’t say it’s frequently used, but probably not rarely either.
Anyway, Meriam Webster has United-Statesian https://www.merriam-webster.com/dictionary/United%20Statesia...
How do you cope with Modern English previously never having been a language anyone spoke or wrote?
Seems like a waste of talent and energy.