2nd battery

Discussions of usable battery capacity pre-suppose that the battery(s) are in good condition and fully charged. A 5 year old battery may have less than the claimed capacity and duration when running the furnace fan and water pump or 110 volt items with an inverter. When rig is parked undriven, not connected to "shore power" batteries will "self-discharge to low levels and their useful life/capacity will be reduced. Keeping battery terminals clean and replenishing electrolyte levels with distilled water and keeping batteries charged, but not overcharged, with a 'smart" charger (Battery Minder) will prolong their useful life. Check battery voltage with a cheap digital multi-meter set on 20 volts DC scale, should read around 13.6 volts if battery is fully charged, around 14 volts with engine running.