the only thing I have any degree of success with removing pencil marks on a muslin type fabric is a "Pink Pearl" eraser. They are a soft rubber eraser. You might try that first to get as much of the graphite off before washing.
But my experience has not too successful if the marks have been on the quilt as long as you say. I have been told the formula of rubbing acohol, water and a drop or two of dishwashing liquid (not Dawn). I don't remember the exact
formula. You gently rub with soft tooth brush dipped in the solution and then blot.