So, unless you want to go down the inverter route, a single battery should suffice.
Presumably just lights, phone chargers, water pump, diesel heater?
Even though we only had the split relay and smart alternator, I never thought the leisure battery was undercharged. I was worried about the mains charger, which was different to yours, as it seemed to keep the battery at very high voltage at times. But yours looks designed for the job, mine was described as a Caravan power supply!
Edit: and if you ever do go Lithium, you'll want to keep the split relay to power the fridge when driving.