Current draw doesn't seem excessive. Voltage at battery with engine running is good.

The only type of battery that is designed to withstand repeated deep-discharge events is a "deep cycle" battery. Car batteries are not "deep cycle" batteries and repeated discharge events like you describe will eventually result in a battery that will refuse to accept a charge.

In your post you didn't indicate how old the battery was that was going flat all the time. If it was new, I'd suspect a defective battery, if not - maybe it had exceeded its time allotment here on earth.

If you've replace the battery with a new one and the problem hasn't returned - you've answered your own question.