You set up something of a false dichotomy between mythology and "actual documents", as the mythology that we have about early Rome survives in actual documents. One of the main sources for early Rome is Livy, a historian who lived in the Augustan period. His first book dealt with the foundation of Rome and the period of the kings; his 142nd book dealt with events that happened within his lifetime. Thus there is no simple distinction between mythology and history - they come from the same sources. Livy wrote in his Preface:

The traditions of what happened prior to the foundation of the City or whilst it was being built, are more fitted to adorn the creations of the poet than the authentic records of the historian, and I have no intention of establishing either their truth or their falsehood. This much licence is conceded to the ancients, that by intermingling human actions with divine they may confer a more august dignity on the origins of states.

Thus, he recognised that the stories about the foundation of the city should be taken with a large pinch of salt. But where do we draw the line? Since Livy rarely mentions his sources, for any given event of early Roman history reported by Livy, it's difficult to know whether he had a credible source or if he was simply reproducing a myth.

Another problem is that the Romans didn't write history until the end of the third century BC, when the senator Quintus Fabius Pictor wrote an account of the Second Punic War. Livy's books about that war are largely based on Pictor's history, but for earlier events, we have to ask how Livy could have possibly known what happened several centuries before his birth. The Greeks had been writing history for centuries, so he could have looked at Greek writings pertaining to Roman affairs, and the Roman state kept records going back at least to the beginning of the Republic. Furthermore, Roman aristocratic families kept the deeds of their great ancestors alive by recounting them at funerals. There are thus several ways in which information could have been transmitted from the early days of the Republic (or even before) down to Livy's day. The question is how useful these traditions would have been: the oral traditions of aristocratic families would likely have significantly exaggerated their ancestors' achievements, and early Republican records would probably have only recorded very basic information (not to mention, as you say, the sack of Rome which may have destroyed some records). Scholars are divided in their interpretations, and there is an extreme view that all of Roman history before a certain point is basically fabricated. Most would probably accept, however, that some genuine facts have survived from the early Republic, but it's tricky to separate these from the surrounding mythology, and even more so for the period of the kings: we may never know if the kings really did any of the things they did in the stories, or if they even existed at all.