Don't listen to music while you're chillin'.
After you're done chilling in the cove, go for a 20 minute cruise (or do another set) to let the engine recharge the batteries.
Buy a gasoline powered generator for the storage unit, hook up a charger, and let it run for whatever it takes to charge.
I don't think there's a good, simple answer to your problem here. If you discharge the batteries while chillin', and drag the boat straight to storage with no power available, your batteries will live a short life - but maybe replacing batteries once a year is the cheapest way to handle the issue.
1998 Maristar 200VRS