When my batteries drops below 12.0VDC which is approximately 50% charge state I will re-charge them. I usually leave my battery bank connected but the trailer is plugged in to my 120VAC 20AMP service coming from my garage during the winter months. Using my smart mode CONVERTER/CHARGER almost takes cares of the batteries automatically. I will however check the Battery voltages and look for boiling out of fluids.
My batteries will drop below 12.0VDC in a short time being drained by the parasitic items in the trailer if not connected to shore power.
It has been my experience that if the BATTERY drops below 50% charge and not be re-charged right away to at least 90% charge state then it will not perform as expected. Continuing to be drained below the 50% charge state will eventually do harm to the battery and will have to be replaced.
Even if you pull your batteries they will will loose some charge. I would keep a close watch on the DC VOLTAGE levels measured at the terminals. ALot of folks will keep a small battery trickle charge or solar panel keeping the disconnected batteries up to par over the winter months.
Simple Battery Maintenance is must do thing here.
Roy Ken