Complex sentence: He may have been a principal officer of the "Mayflower" and since no more is known of him he may have returned to England on the "Mayflower's" April 1621 voyage or died of the illnesses that affected so many that first winter.
Split into simpler sentences: He may have been a principal officer of the "Mayflower". No more is known of him; he may have returned to England on the "Mayflower's" April 1621 voyage or died of the illnesses that affected so many that first winter.