Battery is BRAND new, must have some drain even when isolated?
Usually there are circuits kept connected even though the isolating switch has been turned! The easiest way to check is with a clamp on DC ammeter round one of the battery cables.
Easy way to check - disconnect earth cable and connect a low powered bulb between earth lead and battery negative. If if lights up with the battery isolator switched off, you have a drain somewhere. Have you got any cables connected to the battery positive terminal other than the cable that goes to the isolator switch? You "may" have some wiring for alarm/immobiliser/tracker that could be causing a discharge.
Battery should not got flat if properly disconnected imo unless faulty. What voltage does it measure? For me disconnecting is not the answer, it should be left on a conditioner/trickle charger.