Examining the lack of heroism in a literary piece that has historically been viewed as the epitome of heroic action is a daunting task and I applaud your choice. When you address the lack of selflessness, you are actually addressing the characters' choices that will allow them to benefit. These choices may be conscious and/or unconscious choices that enable them to rise above their current conditions so that they may succeed in whatever endeavor that they may be undertaking.
Specifically, the very actions that brought about Arthur's birth were the self-serving actions of King Uther. Then the baby was whisked off by Merlin to become the ward of Sir Ector. Did this removal of the child Arthur create the ultimate demise of King Uther? Were his actions from his horse-drawn sickbed the actions of a hero or merely the actions of an old man who could not accept his impending death?
Make certain that as you address this topic you remember to show why the action that most view as heroic are merely ways in which that character chooses to better himself and his current situation. When Arthur pulls Excalibur from the stone, wasn't his action just the lazy way that a young boy chose to get a sword for his master? Was it really some extraordinary feat of heroism?
When Merlin tried to assist Arthur in his various battles, wasn't this an attempt by the wizard to gain Arthur's trust in order to prove that Arthur could only survive with his military genius?
Many of the acts of bravery performed by the various knights were designed specifically to gain wealth and praise from their king or the favor of a beautiful damsel. Neither of these reasons would prove to be examples of true selflessness.
As an evaluator of such persuasive pieces like the one you intend to write, I always ask if the writer has thought about the arguments that may be presented by the opposing views. Make certain that you address these concerns to prove that your view has merit and should be accepted as the preeminent view.
Consider the following as you write your paper:
When you describe the actions of Gareth against the Red Knight, you may wish to describe how that he may have chosen to battle the magic knight to prove his worthiness to Dame Lionesse - not as the actions of a brave knight.
When Mordred convinces England to turn against Arthur in the final book, his promises to the war-weary citizens are definitely used by a self-serving character to gain personal glory.
After the final battle when Bedivere supposedly hurled Excalibur into the lake, his lies proved yet how another character chose personal gain over the selfless act of a true hero.
You may also wish to approach the subject as the very idea of heroism (selflessness) is in fact a selfish motive by the characters. If you choose this approach, you will need to convince your reader that heroism cannot exist as an attribute of any of the characters because it cannot exist at all. It is merely another way that the characters choose to better themselves in the eyes of their peers.