Was planning to run a ARB fridge, and just use as a aux and to keep my main battery charged all the time
I'd plan on mounting the battery in the cargo area near the fridge. Just get a dual battery kit from Blue Sea Systems, a couple breakers, and run some 4awg wire to the rear cargo area and go from there. You'll have lots of power available in the cargo area where you'll need it for extra chargers or on-board air if you'd like later on. That's what I did with mine and I have 60A of power available in the cargo area now for my fridge, chargers, etc.
I'll say that I currently run my 95L Dometic Dual-Zone with a single AGM battery with a capacity of 94aH. In 65-85°F ambient temps across CO, my single battery ran my fridge for 9 days straight but the engine was run daily while we traversed across the state's backcountry. - Now that we're back in Texas, I don't want to take any chances since the higher temps cause the fridge to draw more amps and I plan to park it for a few days at a time at the beach, etc. I'll be adding a soft solar panel and secondary battery now, but since I ran power to the rear I can just place the battery and solar input in the rear and wire it up to charge both batteries.