^ absolutely right. In any application (low or high voltage) there's a drop over distance in the available volts under load. The thinner the wire, the worse it gets. The problem isn't as apparent when there's no load. Easy to test: grab a halogen spotty (100W = 8.3A) and hook it up to the other end of your cable. While it's lit, measure the voltage at both ends. The voltage drop (for that load) will be very apparent if the cable is over 3m long.
That's important, because even a non-smart alternator only puts out enough volts to charge the cranker. Once you add voltage drop, the battery on the far end of the cable is not going to get enough power at all, and it will leave you quite short for your stay.
Don't despair - there's still a way to do it cheaply (and effectively). Don't take shortcuts with the cabling though - the 8Ga fig8 cable from Jaycar will do the job nicely, buy about 8m of that (a little more is better than not enough). Connectors should be Anderson plugs, they grip each other firmly and you can't orient them the wrong way.
Grab an oversized relay - 80A or better - and connect as follows:
* Pin 86 (small wire) to battery negative
* Pin 85 (small wire) to any accessories-on source in the engine bay. Test that the source switches off when the starter is engaged.
* Pin 30 (heavy cable) to a heavy fuse holder then to battery positive
* Pin 87 (heavy cable) to the rear.
That's all you need. When ACC comes on, the cranker will be connected to the auxiliary batteries. Because the ACC feed is disconnected during start, the power will NOT flow from AUX batteries to the cranker.
The caveat here is that the aux batteries will probably never charge to 100%. They'll get some power, oh for sure - but they'll NEVER get full. It's just not possible (with the direct connect described).
There is another way - and you can remove that relay (or put in a switch on Pin 86 inside the cabin) AND get the batteries fully charged. It's only a little expensive but has a nice side-effect. Incidentally, this is the way I was doing it until I permanently mounted a solar panel on my canopy.
Same deal as before, run heavy cable to fuse then down to the rear of the car. Now, invert the power. Yes, buy an inverter. Powertech range is acceptable, the Aldi ones are also good if you can get one - you need one whose output is electrically isolated from the input (the documentation should state this). Plug both a multi-stage battery charger AND your fridge into the output of the inverter. Connect the battery charger AND the fridge to the 12V aux batteries.
Now, this is where you get not only the cake, but the icing too. Modern fridges will, when they sense 240VAC, use the AC feed in preference to 12V. It's not because it's more efficient - all of these fridges will convert the power back down to 12V. It's because they do their best to keep their load off the battery. Now, with an unloaded battery, the battery charger will see a clean, unladen battery and will charge it completely and perfectly.
Which battery charger? It has to have the following functions:
1) Auto start when mains power comes on (some don't - particularly Aldi ones)
2) Auto sense battery state (almost all smart chargers do this)
3) Auto float the battery once charged (most smart chargers will, simple chargers won't)
A charger like
this one from Jaycar is good enough. Most of the stuff I saw at Supercheap was notreallycheap.com.au (or maybe wayoverpriced.com.au) and Rays Outdoors only has
this one which is adequate. I didn't find anything at BCF worth having. Forget Bunnings, they only sell the Projecta stuff and although they claim 2.5A charger at about $80, it's really a 1.6A charger (1600mA).
