adding 3rd battery

Hello comrades

I have a Calcium SLA battery as a starter in the TD Jack. I have a 70 a/hr AGM as an auxillary under the bonnet, isolated by a Pirahna 150 amp unit. I used to have a 120a/hr spare AGM in the back which I just kept charged and chucked in the back when going away when the possibility of staying stationary for 3 or 4 days occurred. It was not charged via the vehicle in any way. I have just made a fridge slide/drawer unit and the 120a/hr no longer fits. I have purchased another 70 a/hr AGM and am wanting to link the 2 auxillary batteries together, both to be charged via the vehicle.

That was a bit of history, now for the question. Is it possible and advisable just to link the 2 auxillary batteries in parallel via 6mm wire, positive to positive, negative to negative, and have the isolator connected only to the battery under the bonnet, and to draw power only from the one under the battery? I am thinking with my limited 12v knowledge, all I am effectively doing is creating a 140 a/hr battery, but just wanting to check. Also, is there a need for some sort of fuse or circuit breaker in between the two? Should I run negative wire from battery to battery or will be earthing it to the chassis be sufficient?

Thanks in advance.

D-Jack