Although it's called the Song of Roland (La chanson de Roland) it's not, of course, a song in the sense we use the word today. It's a long piece and would require a particularly trained memory for it to be passed down orally. A bit like expecting someone to recite, declaim or sing their way through a short novel. I studied it at university which is, sadly, a long time ago. I can't imagine it would have been in frequent demand by audiences, (except very warlike ones!) so the incentive to pass it down orally would have been reduced. It does appear, however, to have been well known at the time, and as you're talking about a largely illiterate population, before the days of printing presses, its main method of transmission must have been oral at some stage.
A traditional version would, presumably, have become much reduced and simplified and, if it exists/existed at all, would have been in oral currency in northern France. Although the links suggest it has been preserved in different forms in Sicily (the Norman connection again).

Anne