No cranking, radio, interior lights or headlights working.

hey guys, new to the club. i see you guys helping around the thread alot and was hoping you could help me with my first car. i have a 75 duster and its been acting up since i got it. there would be days when it simply wouldnt crank and accessories like radio and lights wouldnt work then they suddenly work again. that was a couple months ago now something similar has happened and it hasnt been fixed for days. ive replaced the starter relay and the fuses, and made sure the battery was good. tested with a new one but still nothing. hope you can help out. any advice questions are appreciated