load sharing dc to dc LiFePO4 battery charger

I am trying to put in a secondary LiFePO4 battery in my car that powers a car refrigerator. I want it to work like a UPS that switches to battery when the car is turned off but while turned on, it charges and powers the refrigerator simultaneously.

From what I was reading, a standard parallel circuit to a DC-DC charger can cause problems.

I am looking for a product that a similar design as the diagram below.

What product do you guys recommend that can do something like this?

Thanks in advance.