It would help to know what brand and model your rig is. I would GUESS that the switch disconnects the house battery during storage or non-use. The "auxiliary battery disconnect switch" is usually located in a control panel in most RV's along with switches and LED indicators for water pump, holding tanks and battery charge levels.
12vdc alarms and appliances that are connected to the house battery, plus self-discharge will cause the house battery to lose charge during storage/non-use. Your RV generator may be connected to your house battery or starting battery for starting it. There may be a momentary "emergency start" switch on the dash that connects the starting battery and house battery in parallel to start the engine or the RV generator when either battery is low. The switch in the battery compartment should be on when connected to shore power and for the house battery to charge from the converter/charger when on shore power, or from the engine's alternator. Recommend you get a cheap digital multimeter from Harbor Freight and set it on the 20 volt DC scale to check whether your new house battery is getting charged by the converter/charger ( should read 13+ volts) when on shore power or (should read around 14 volts) when being charged by the engine's alternator when engine is running. Older converters charge slowly, the engine's alternator charges house battery more rapidly. Your house battery may last 2-3 days of normal use when not connected to shore power unless you run the furnace, it's blower will usually run the battery down overnight. Learn as much as you can about 12 volt electrical system including care and feeding of batteries. Keep battery electrolyte levels above the plates using distilled water.
