Question: Caravan & Camper Battery Charging

I wish to install an additional battery (100ah AGM) to run my Waeco fridge in the rear of my twin cab ute, which is used to tow a caravan. The van is fitting with 2x 100ah AGM batteries with are charged via the ute while travelling and solar/240v when stationary. As there is an isolator already fitted for the van batteries, do I install a separate dual battery system (isolator included) for the ute or use the existing battery isolator.