Right time for an update (apologies for the photos they are taken with my iPhone not my SLR). Thanks for all the comments and the interest in the thread.
The second battery is in with a National Lunar solenoid
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I found the NL monitor to be basically useless so I took it out and put in a simple voltmeter with an inline switch. Its connected directly to the battery although there will be some voltage drop so its not totally accurate but its enough to give me an idea of how much juice there is left so it will do the job.
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We ripped everything out of the back seat area and put in the new frame, fridge plinth and inverter mounted out of the way under the plinth
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Then the new Frontrunner water tank went in
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then the fridge went back in
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and finally the seats
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The water tank was bloody expensive for what it is but it has to be said its a super efficient use of what would otherwise be dead, or nearly dead space. The 41l water tank will be supplemented with a 20l jerry can which will give us 60l in total and will also be used to haul water from lakes and rivers up to and into the water tank. I'm not yet decided on wether to just put a bit of hose onto the valve and leave it at that, to wether to plumb in a tap with a locking lever mounted by the seat.
I've also ordered an ARB winch bumper which should be here soon.
Still lots to do but getting there.......