Part of what you're asking can be traced to the idea that the gods and heroes of legend are based on real people. This concept had an early proponent in the Greek, late-fourth-century BCE writer, Euhemerus, giving his name to this approach to myth and legend: Euhemerism. Folklorists generally regard the idea that there was an actual basis for most oral tradition as barking up the wrong tree, because the original “real” event behind a story is usually elusive and searching for that core is a futile exercise. In addition, research into how stories began usually concludes that they emerge in a rather spontaneous way, typically without an actual incident to inspire them.

The idea of "the truth behind the legend" has been persistent if not always clearly stated. The Icelander, Snorri Sturluson in his Prose Edda of the thirteenth century wrote about the real people who he believed were behind the Norse gods, but we can wonder if Snorri wasn't directly drawing upon Euhemerus since he had apparently read some classics. In other words, we don't know if Snorri was responding to a written tradition with Classical roots or if he was responding to folk belief. My inclination is to see this part of Snorri's writing as responding to the former.

Seeing the questions asked and the responses to my answers for nearly 9 years on /r/AskHistorians has persuaded me to conclude that there is a modern belief that legends have an element of truth within them, but where and when that belief originated would be difficult to pin down. More to the point, I don't know.

The thing about Euhemerus - and even Snorri - is that there is no reason to believe that they were reflecting folk belief. Their writing may reflect more of an intellectually derived conclusion. Pre-modern folk believed in legends at face value; they would not normally look at their own legends and say, "well, I don't believe in this, but there must be an element of truth at its core." Instead, they would say, "I heard this story, and I believe it to be true - not something at its core, but the whole thing." At the same time, there have always been skeptics, but they would tend to accept or reject a legend in its entirety - not in some circumspect way. This is even the process that goes on when modern urban legends circulate - at least for the length of their life span, which can be shortened because of modern media.

The idea that I have been recently proposing as I answer questions on /r/AskHistorians is that there is modern folk belief that has been masquerading as good, solid social science, namely that there is an element of truth behind legends - especially those that were believed in former times. I feel confident in saying that this observation has been around for a good half century, driving many inquiries into fabled events in the past: the story of the Great Flood cannot be mere oral tradition; there must be a real catastrophe behind it. Or, Arthur can't be just a legend, there must be something behind it. Dragons can't be just legendary, they must exist because people found fossils or we all have some sort of genetic, embedded memory of the danger of deadly snakes. Most of this is advanced as pure speculation, but it finds grounding in the accepted "fact" that "all legends are based on something."

That's not to say that something can't be behind a given legend, but we cannot assume that there is always something there. I have found myself increasingly concluding that the "fact" that something is behind every legend is as flimsy as most of the speculations that are imposed on ancient myths. Strip away this "fact" - let's begin with the assumption that there is not necessarily anything real behind legends - and then the speculations that have been put forward suddenly appear to be shockingly thin.

Then there is the question as to whether anyone has advanced this idea that there is a modern folk belief in the core veracity in legends. For that, the answer is "not to my knowledge." I do not know of anyone in the field of folklore who has made that observation - but there may be something out there I have not read (I have not read everything, nor have I looked for this). This concerns me, and I am considering advancing that idea before one of the folklore societies to see if there is any response. I am considering this thanks to your question!

My emerging idea is grounded in the "fieldwork" that I have done for almost 9 years - as I have said: I answer questions at /r/AskHistorians and I observe what people are asking and how they respond, and I am coming to the conclusion that the group of "folk" known as redditors consistently advance this belief without any grounding in fact. That's not a criticism! Everyone has folklore. Shining a light on it is often difficult and not always well received! That said, it appears to me, that this idea about legends is folklore in itself, a bit of meta-folklore, a folk belief about legends as a genre, and especially about classical mythology and older legends.

I have encountered the belief that all legends are grounded upon some truth for decades - and I believed it not so long ago. What I realized answering questions here is that people repeatedly put forward someone's theory that "explained" some legend, and then asked if this is valid. I repeatedly found myself answering that these "explanations" were consistently based on unproveable speculation, making them more a matter of belief and faith rather than hypothesis and theory. In addition, some of these speculations attract a great deal of emotional defense from redditors, indicating that they are invested in this belief - and that it is sufficiently widespread to be regarded as a folk belief.

That's where I stand on this at the moment. I plan to continue my fieldwork as I observe this remarkable place called /r/AskHistorians.