Yes, when you need a comparative or superlative form of "full", the correct forms are "fuller" and "fullest", but "more full" and "most full" are sometimes valid alternatives.
Some phrases, such as "to the fullest" or "to the fullest extent", would sound odd with "most full", so it really depends on what you mean to say.
EDIT: I would say "a fuller understanding", not "a more full understanding". To me "deeper" and "fuller" are different aspects of understanding.