Bama,
All good advice above.
Consumer grade deep cycle batteries have less tolerance for discharging below 12 volts (about half power). So it sounds like you have exceeded that by a good margin repetitively.
Keeping a battery in use beyond its time is adding an unhealthy stress to the alternator.
Besides an ACR/VSR providing automated convenience it also serves to protect your alternator from an inordinate load. However, it will encourage deeper battery depletion and increase your dependency on shore charging.
So first get enough battery capacity so that you do not have to discharge the stereo battery bank any deeper than is recommended. And then match that battery capacity in combination with your deepest occasional discharge to the right capacity shore charger.
If you do all these things correctly an alternator upgrade is rarely needed and you should get multiple seasons of good performance from your battery(s) investment.

David
Earmark Marine