The alternator must be connected to a lead-acid battery, plus wire to the plus side of the battery minus wire to the minus side of the battery. I don't know what color wires you have.
I assume that you intend to connect the dc-dc converter to the lithium battery. In this case, the dc-dc converter is located between the lead-acid battery and the lithium battery. It charges a lithium battery from a lead-acid battery.
Check if you have a splitter, which split the electricity between the engine battery (lead-acid) and the house battery (lead-acid) in the new scheme, you don't need it and it "uses" electricity unnecessarily.