You can remove or disable the isolation solenoid and install a lithium BIM 225. Use heavy cables from both large terminal to the BIM.
Otherwise, disable the isolation solenoid and just connect the B to B charger to each side of it. No need to find the alternator output wiring. You want that to contionusly charge the chassis battery.
Doing it this way makes converting it back if you trade it in and keep the batteries.