Yeah, I remember coming across USian fairly regularly in the 2000s, but I can't say I've particularly noticed it in the last decade.

I've never heard of USAnian before, but that doesn't mean it isn't used by some people, just not the ones I interact with.

Before USian, I'd come across Merkin, but usually British writers using it in a mildly derogative sense because of the word's other meaning.