America is mentioned infrequently; either the Continent as a whole or the United States of America.
In Missee Lee Captain Flint claims to be Lord Mayor of San Francisco, but one of the pirates apparently knows that American cities do not have Lord Mayors. "You come from Melica" asked the Taicoon (ML7).
Captain Flint and Timothy use the American term Howdy, suggesting they have been to America. Nancy says that Captain Flint may just be strolling up here to say howdy ..... (PP28). Titty also uses it; presumably learnt from Captain Flint. John says that Titty always says "Howdy" to him (Polly), even if she's only been away ten minutes (SD33).
During the fire, "Squashy Hat" says it looks like a Prairie fire, also suggesting that he has been to America (PP32).