Many thanks to Drivesafe, neutralfibre etc for all the electrical advice on wiring the dual bat system.
So, here are the pics with some explanations.
This is the setup. I initially found a 60L water tank on a wrecked caravan at the dump (SCORE!) filled some breather holes and converted it to sit on its side. Then using some 12mm ply i constructed the frame for it to sit in. The frame also houses my aux battery on the right.
The fridge slide was also made from dump material. I found some angle iron and chopped it to create the two components of the slide; the draw and the base from which I attached some roller slides i bought off ebay. A few hooks from BCF and she's done.
I also used some linatex rubber when it was bolted up for some shock protection.
Water tank and battery in background.
Using a small inline pump (BCF) and some garden hose I wired the pump up to the tank and whola! Pressurised water.
A 30A SPDT toggle switch is used to turn the pump on/off.
THe battery system consisted of a simple 200A continuous duty solenoid to do the switching between two Supercharge allrounder 80AH batteries. I wont go into too much detail on the wiring but two things to note is that:
1. The solenoid is powered off an ign wire that goes to teh alternator and I included a switch in this loop so I can decide if i want the solenoid to connect or not. This eliminates one good aux battery masking a bad starter battery (as long as I remeber to flick the switch and check every now and then).
2. I ran the neg wire from the aux battery all the way back to the main starter battery so i made sure i had a good earth connection.
3. All battery-battery wires were min 13.2 mm2 (i think i used even bigger xsection area though). All purchased form Jaycar.
Overall the voltage drop was measured to be 0.1V between the two batteries. I thought pretty good.
This is the switch that turns the solenoid on/off when the ign is on.
Another little trick, I wired a $10 battery indicator to the aux battery in the back and put it in the glove box so i can check the battery charge anytime.
Lastly, the cigg socket in the tray. I purchased a dual socket off ebay and screwed it into the tray. The best part is that it is located such that all wires and fuses are located behind the rear tail light so it looks real nice and tidy.
Thats about it. Hope this gives some people ideas. Goodluck
Dane