It is very easy to see that certain right-leaning or obsolete cultural narratives are false. Take "rugged individualism" or "melting pot," or "Yankee ingenuity." We can dislike them because they lean right or because they are false, or for many other reasons. What I am suggesting is that even those myths we happen to like are also mythic, based on tropes, and poetically constructed. None of these cultural constructions are simply given.
So there are at least three issues:
1) Ideological causes and effects. Take the myth of Southern "nobility" in the American civil war. It has certain causes, and certain effects. Working either way we can see that it is ideologically pernicious.
2) "Truth" or "falsity." Those are the kind of claims susceptible to disproof on more or less objective grounds. We could point out that people who rely on the myth of rugged individualism direct industries dependent on government subsidies.
3) Tropological structure. That is the way the particular myth is constructed in terms of a synecdoche or metaphor or whatever. Take Nixon's question of whether "it will play in Peoria." The geographical middle of the country is supposed to represent a sort of averageness. The mid-West is the synecdoche for the country as a whole.
I have been using words like myth, fiction, trope, and narrative, often interchangeably. It would behoove me to be more rigorous.
Myth refers to a certain cultural / ideological effect, as in Barthes' Mythologies. The myth is something cultural that is seen, falsely, to lie in nature itself. Its ideological nature is hidden. It seems denotative but is really connotative.
Myth and fiction imply the constructed nature, and also implies a certain falseness.
Narrative implies a narrative version of a fiction or myth, a story about how things came to be. Narratives are often fictional, but they are always human, cultural constructions. Even "true" narratives are not simply given by objective reality itself.
Master-narratives (or "metanarratives" meta-récits or metarrelatos) are larger narrative structures in which individual narratives are inscribed.
Trope refers the metaphorical or metonymical structure of the particular construction of reality. Master tropes are those that are seen at work on a larger scale.