Americans have the filter, they are an exception to the rule.
Even that isn't totally true - my 2004 American Tradition has none.
Some American coaches have the furnace mounted in the basement with the air return on the floor above. Those have filters. The ones with the furnace mounted higher were not provided with filters. Apparently they don't ingest as much dust when installed that way.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL