You didn’t replace the battery and alternator because you checked them and found them defective, did you? You can do so, now. How? You can go to an auto parts store and they will test the battery and alternator at curbside, for free.
After the checks, you can check the wire connections from the battery to the power distribution center, and the grounds from the body, engine, frame to the battery negative post. You might need someone to show you how to do the checks…a friend, or other instructor.