So lets see what we have figured out so far.
Your alternator puts out only 13v, rather than over 14 like normal.
You’ve used multiple batteries with no change from what I can see.
And you are having more drains of power than what’s considered regular.
So…
Be simple. You can remove the battery connections, after parking. Measure that voltage. With that said, compare it to what it’s like connected when you drive next.
Becuase this way, you know if it’s from the starter or something thats drawing power after the car is done being used for the day.
Let me know what happens after you have finished both comparing, thanks.