96 Chevy K1500 5.7 sudden loss of voltage

I have a 1996 Chevy K1500 5.7

New battery, bought a new alternator roughly
6 months ago. I had the issue at first around 6
weeks ago. Take off down the road and
suddenly no electricity. Radio quit playing,
lights were too dim to see anything at all, and
truck defaulted to limp mode. Bought a new
battery and the problem was seemingly
solved. Went to pick up a battery for my lawn
mower and when I walked back out of the
store truck wouldn’t start. Diagnosed it in the
parking lot and decided it was probably the 25
year old starter. After a new starter and a jump
it started no problem. On the way home
however it started the same ol song and
dance. No radio, no lights, no voltage. Where
to start?