Maybe it would simplify things if I just went with 1 or two more batteries that match the existing ones. Ideally I'd just wire in an anderson connector to be able to plug in the external batteries.
Are there any complications there or can it be plugged in 'live'? I could at least turn off the power to the camper/solar with disconnect switches but that final connection would have to be live it seems... unless I need another switch between the batteries and the anderson connector which I really don't want because it would be exposed to the elements somewhat.