If you have standard quarter-turn on/off switches with removable red handles, add another on/off switch, like your existing starter battery switch. Wire it between the domestic positive and the engine side of the existing starter battery switch. Remove the red handle from the switch. If your starter battery is flat, remove the red handle from the starter battery switch and put it in the emergency switch and switch on. The starter battery will remain isolated so it can't drag down the voltage of the domestics. Your engine will then start from the domestic batteries. The starter battery will recharge as normal from the alternator. Remember to replace the red handle in the normal starter battery switch later.
