The right DC-DC charger will have a sense wire that detects when the truck battery is at a sufficient level (being charged) to allow the DC-DC charger to draw from the truck battery. Some DC-DC chargers don't even need a sense wire.
A DC-DC charger is a one-way circuit. It will not draw from the LiFePO4 battery to charge the truck battery. In my side-by-side, I have an isolator to keep the secondary battery charged without drawing on it. The isolator has a button to allow the secondary to join the circuit for cases where the primary battery is not enough to start the engine. With a DC-DC charger in the circuit, the isolator would not be able to join the LiFePO4 battery to the truck battery to charge the truck battery. You would need a separate circuit that bypasses the DC-DC charger.
A DC-DC charger is a one-way circuit. It will not draw from the LiFePO4 battery to charge the truck battery. In my side-by-side, I have an isolator to keep the secondary battery charged without drawing on it. The isolator has a button to allow the secondary to join the circuit for cases where the primary battery is not enough to start the engine. With a DC-DC charger in the circuit, the isolator would not be able to join the LiFePO4 battery to the truck battery to charge the truck battery. You would need a separate circuit that bypasses the DC-DC charger.