We do alot of Dry Camping and seem to run low on power, so we pulled the single battery out and bought 2 12 Volt Batteries. They are the Deka DC24 Battery. We also picked up some 4 gauge battery wires already made ane another battery box.
Our Parts:

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The batteries:

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I Made sure to drill holes in the bottom of the box to allow water to drain

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Placed the two boxes and batteries on the frame:

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Wired the batteries in Parallel and hooked up a switch to shut off the pack:

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Dressed up all the wires and closed them up :

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Connecting in Parallel
When
connecting in Parallel you are doubling the capacity (amp hours) of the
battery while maintaining the voltage of one of the individual
batteries. Use a jumper wire between the
positives of both batteries and another jumper wire between the
negatives of both batteries. Connect your positive and negative wires to
the same battery to run to your application. 
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Important notes:
When connecting batteries in a pack there are some important things to
keep in mind - - Find out the requirements of your application. For
example: Don't double the capacity on your Power Wheels vehicle if
you're not supposed to...you could burn up the engine. Follow the
recommended guidelines for your application. - Don't use two different
chemistries when connecting a pack. Usually the voltages will be
different, but more importantly the charge rates will be different and
the capacities may be different, thus resulting in a shortened life
span. - Try to match capacities as much as possible. When connecting
batteries in a pack you should try to match the capacities as much as
possible to avoid discharging one battery quicker than another. A pack
operates at a combined voltage so your one cell that discharges quicker
will likely discharge deeper than it may be able to recover from.