Duel Battery setup

Ok then Dual batteries. I have a D-max dual cab and want to put a second battery in the back for the Engel etc. So, second battery being charged after main engine battery is charged. An isolator between the two so the engine battery is just for starting the engine. Second battery will be deep cycle.
thanks

Sorry Geoff I was having a lend of you a bit.
I have the same vehicle.
Firstly, because after a start the engine battery will be fully recharged after a very short time you can immediately have a heavy continuous relay connect to the aux battery.
If you have the fridge in the cab, with the seats out, you can feed power from the Aux in the tub into the cab through the fuel filler cable release grommet, LHS lower corner,

I have a 105 Full River AGM for the aux battery.
I use a heavy continuous relay which can be switched on after the engine has started or before if necessary to assist if main is failing.

I also use a manual battery isolator connected in parallel with the solenoid so both batteries can be connected all the time, so the solar on the roof can keep both topped up.
I use Welding cable from the the solenoid/batt isloator to the rear of the vehicle so it can also charge the van 120ah AGM via anderson plug.
The weld cable has a connection into the tub through the lhs oval grommet and to the 105ah in the tub box.

While charging I get 14.4v at the aux battery, it is well earthed to chassis.
Really good earths are used and the body/chassis and main batt negative all have extra earth cables fitted so hardly any voltage drop occurs in the whole circuit.
All works well, no fancy DC/DC anything.

The charge to the aux is selected by a system. either always connected when key is on and therefore connected when starting, or selectable so I initiate the hookup as required. Always disconnects when key is off. Custom switch system used there.

Hope this helps.
Ross m