Welcome to the forum.
Simple answers:
* The larger battery on the right hand side of the vehicle is the starter battery
* The engine needs about 500-550A to turn over so the larger battery on its own is good enough
* Isolating the second battery is a great idea but move the brake controller to the starter
* Powering devices - especially fridges - in the tub will NOT be as successful as you'd hope
The trick with 12V supplies is that the voltage drops dramatically on light duty cables when there's a load on them (that's the important bit - voltage is fine when there's no load). Even the 4.5A typically drawn by a Waeco/ARB/Stirling (all the same compressor) fridge is enough to bring the voltage down. These fridges have a circuit that monitors the voltage and when it falls below a certain level (Waeco = 11.75V, others would be similar) the fridge shuts the compressor down.
While that makes sense most of the time, it doesn't when it's due to light-duty cabling. What ends up happening is the fridge stops drawing power, the voltage rises again, the circuit sees a good voltage, brings the compressor back on and drops the voltage. This happens every 30 seconds or so - and will very quickly kill a battery.
There are two ways to fix it:
1) Use a shorter cable. Voltage drop is a function of cable size and cable length - generally anything over 3m tends to start showing a larger drop. Short cables aren't possible when your battery is in the front and the fridge is in the back - you're looking at a minimum 6m of cable for that.
2) Use a heavier cable. Yes it's more expensive but so's throwing food away because the fridge hasn't kept it cool, and worse the battery that keeps being killed by over-discharging it ... I use 8Ga fig8 cable from Jaycar and don't have a problem.
There's one other issue you need to be aware of. The second battery in your engine bay is a cranker battery. These batteries are designed to dump lots of power for a short period of time and be charged up again straight away. They are not designed to be discharged slowly over many hours and not charged again for a day or more - this will kill the starter battery.
In your budget for changing things over, add a new battery to the mix - there are many choices, but Gel is one you should NOT consider. AGM is best of the lead acid batteries, LiFePO4 is the ultimate at the moment in portable power sources but you'll have to work on some kids so you can sell 'em to buy the batteries.
Lastly, the isolator - up that close to the battery, you could just use a VSR like
this one. No need for special trickery, it will charge the aux battery fairly quickly (it's really just a switch that connects the two batteries together). If you move the battery to the tub, you'll want a proper DC-DC charger like the C-Tek D250S which will also take a solar input and use the best of either solar or alternator.
Hope that helps a bit!
