A good part of the problem here is the word "myth" (let's set aside the word "mythology" which means the study of myth).

When it is not used in the context of ancient stories, we often use the word "myth" as a point of derision (see the text that I will add as a reply to this one). We say "that's just a myth" to discount a story that should not be believed, with the implication that anyone who would believe such a story is a fool. This is a problem of terms within the English language.

If we consider the narratives that are represented by ancient myths, we can quickly see that they are what folklorists call "legends," - stories generally told to be believed. Legends come in a variety of forms including heroic, historical, and etiological (describing the origin of things). Most cultures have forms of legends in their popular narratives. It appears that many of these from ancient cultures inspired authors who codified stories that circulated orally, giving us the written texts that we now refer to as myths or that we study under the rubric of "mythology."

I always discouraged my students from using the term "myth" to describe living belief systems and their narratives because of the judgmental nature of the term. Once I had a student who asked about Native American myths, and I cautioned against using that term. As it happened, a graduate student who was a Native American was in the class, and I asked her how she felt about this issue. She pointed out that her grandparents and many of the people in her tribe believed in those stories and that she found it offensive to use that term. Imagine the difference if we were to talk about the "Resurrection myth" as opposed to the "Resurrection story." The difference becomes immediately apparent, and it comes down to a wish to insult or to be respectful.

Some of the issue around your question about Hindu Nationalists, then, can be diffused is we set aside the term "myth" and discuss the origin stories - or legends - of the Hindu people. Can these be regarded as "history"?

A similar issue can be found in the origin story - or etiological legend - found in the Book of Genesis. I frequently see questions on this and other subreddits asking if we shouldn't regard the story of Creation and the Garden Eden as nothing more than a myth, deriding those who believe. But if we are to apply the same standard, reserving the term "myth" for ancient cultures that no longer have believers, than it is unfairly cruel to apply the term to any part of the Bible. Genesis describes an etiological legend that some take to be literally true (fundamentalist Christians, for example), that others take to be allegorical, and still others take as a quaint story that should not be taken seriously. I have heard scientists point out that Genesis can be taken as a surprisingly accurate description of how the solar system and the planet earth and its life came into being. If we allow for divine days to mean billions of years, we can even fit it in as a true if not very detailed version of how the world, life, and humanity came into being. Can we see this origin "myth" as being in fact a form of history. The answer is certainly yes, if we allow for a need to make adjustments out of respect for the belief system of those who see it as such.

Many indigenous people internationally are faced with the same sort of question - Hindu Nationalists are not alone in this process. Traditional narratives that describe the origin of things and early heroic periods are being "fitted in" to a past that we understand from geology, biological evolution, or history of ancient times. This sort of merging of history and traditional narrative is an interesting, dynamic cultural process, and it is becoming increasingly common internationally as people attempt to reclaim their cultural heritages and to reconcile them with other concepts of the past - from the scientific to the historical.

How do I feel about that? I find it rather exciting as long as it doesn't strike a militant, fundamentalist tone that forces that perspective on non-believers and on the education system that includes non-believers, particularly in a way that discourages the story of the past in a rigorous, analytical way. Too often that is the next step, and I find that to be harmful for society as a whole. But perhaps that's just me. I'm an old fart, and sorting out the good and bad of it is something left to the vast majority of the rest of you who will be forced to navigate your way through the rest of this century, which has started with so much woe.

I hope you will get a fuller answer soon, but as you are waiting I can recommend some earlier threads.

When it comes to defining mythology, our own folklorist u/itsallfolklore has written a lot about this, for instance here, here, here, here and here. Another of our flaired users, u/KiwiHellenist, has written about if truth can be found in myths and epics on his blog.

When it comes to the Mahabharata and the Ramayana specifically, this is discussed a little here by u/MaharajadhirajaSawai, here by u/artfulorpheus, and more briefly here by u/Tiako. If you are interested in the broader history and development of Hinduism, you can read this by the first-named user and this by u/Trevor_Culley