If you want your extra leisure/auxiliary battery to get fully charged you have to have a 12V to 12V charger.
If your starter battery and your extra battery is LiFePo4 then you most certainly have to have a charger to save your alternator.
The resistant in a Lithium battery is a lot less than a Lead acid and will withdraw excessive amount of power from the alternator
I have found two solutions for Dc to Dc charger here below
|2000W 12V Pure Sine Wave Inverter Charger w/ LCD Display - $699.99
The Renogy 2000W Pure Sine Wave Inverter Charger can be used for multiple applications when going off-grid. A power inverter is an electrical device that transforms the DC power stored in a battery bank into standard household AC power for a user's electronic needs. This device can also charge and maintain a battery bank when connected to shore power and is now compatible with Lithium batteries. With a low transfer time, power remains uninterrupted when switching from solar to shore power. This creates clean and silent energy without having to worry about losing access to connected devices and appliances. The Renogy 2000W Pure Sine Wave Inverter Charger is a perfect back-up power source!