Let me start by explaining how it all works. The wiring is really nothing clever.
There are three batteries. Two under the bonnet and one in the back. Think of the passenger side battery PSB as the central point; everything revolves around this. The PSB really only does the cranking. OK it does have a couple of sneaky bits running from it but nothing that could drain to earth and flatten it. The alternator is connected to the PSB. So when I fire up, the PSB gets the first charge but at the same time, power goes to the CTEK system for charging the rear leisure battery RLB. At this point, the second battery under the bonnet is getting nothing. This is the Aux Power Battery APB. This battery runs everything that came as standard on the vehicle such as headlights, demisters radio indicators and so on. it also runs a few additional USB chargers and things, so it is ENTIRELY possible that you could flatten this. I never have, but you could. It also runs the solenoid for the starter motor. So if it was totally dead, the car wouldn't start - ahh, but it will, just wait a bit.
Now after about 5 mins the time on the split charge under the bonnet kicks in and connects the PSB to the APB and the alternator (120amp) gets to work topping up the APB which if parked up, might have had quite a hit. The PSB though as it's done nothing more than 1/4 second's work to flick the engine over is already full. Now, there is a feature on the split charge to be able to manually connect the PSB and APB by pressing a button. I generally do this before I start the engine and this way the APB doesn't have to wait 5 mins for supper. If it's dark and the headlights are on, fan, radio etc, then that's another 5 min drain on the battery and if it's all stop start stuff then the timer would never actually kick in if you follow. OK the other advantage of this button is that IF the APB was flat, connecting the two front batteries in this way gives enough power to fire the starter solenoid. If by some freak the split charge failed, or the control button, you can use a single jump lead to connect the two directly and start the engine. I carry a small floing lead to bypass the whole split charge if I want to and take it back to being two batteries in parallel. The only reason I do have the split charge is so that any drain on the APB cannot cross and drain the cranking battery. In other words it's ring fenced. Personally I don't like the National Luna split and its timer. I prefer the IBS and the voltage sensing circuit. It closes the solenoid as soon as it detects sufficient current / volts which generally is immediately.
OK, now whilst all this is going on, the CTEK is taking whatever is available and ramping up the volts as high as it can. I am no scientist, but I get the theory of the relationship between volts and amps. It can ramp the volts up, but I believe it reduces the amps in doing so. The rear Optima battery performs best when the amps are pushed in at a higher voltage. So it will be giving somewhere between 14 and 15 volts usually. With the bigger alternator there is plenty of amperage available so it doesn't struggle to charge all three. Gradually the amps will reduce at the CTEK until the RLB battery is full but unlike the front battery charging profile, the volts don't ramp down the 13.8 or 12.5 etc. They remain high. This is how it's supposed to be I understand for optimal Optima performance and so far it's been very good.
Now meanwhile in the front the battery charge is dropping and generally down at 13.8. This is actually affecting the APB a bit and it just doesn't hold charge like the RLB does. So most of the time, I stick the intelligent charger on it when it's up at the unit in storage. The RLB powers the fridge, work lighting, USBs etc etc
Now I do have a (broken) solar panel too and that provided some charge to the CTEK which goes into the RLB. I shall get a new unit and hopefully produce a bit more electricity than I am. The clever thing about the CTEK is that once the RLB is full and the sun ios shining, it will change the direction of flow and actually send surplus power back to the PSB keeping it topped up. But because I have a split charge under the bonnet, once it detects the PSB reaching full, tihe time will start and 5 mins later will open the tap and push power over to the APB too. Awesome. Now because I have never made enough power on the solar I haven't seen it do this. But it should. Have to be pretty sunny though.
So that's how it works. I used the same batteries in the RLB and APB positions so that they were interchangeable. Also the Optimas have central posts which means that can sit in either side of the front battery carriers under the bonnet in the event that the PSB fails. So, in short, there is little chance the my truck won't start but if it doesn't it's not going to be down to batteries.
As to the wiring, there's nothing to tell really. It's all just wired neg to neg, pos to pos coming off the PSB. With a break of course to put the two split chargers in. Other than that, there's nothing more to it. Part of this of course relies on the fact that I have changed my vehicle from 24v start to 12v or else it wouldn't quite work the same.