I'm a little late to this thread, but I was an automation tech for years and have spent quite a bit of time rewiring my old dodge. Here is a rough diagram of how you're going to want to wire the second battery:

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The negetive wire comes from your car battery, goes to the intellec, then comes out the other side and goes to your second battery. the positive wire should be connected directly.

Then, as others described, one of the two little terminals goes to ground and the other goes to accessory power - something that is only energized when the car is running. With this system, both batteries will be in use while the car is operational which allows the alternator to charge up both batteries. When the van is off, only your secondary battery will be used to run the accessories.

You'll probably want to use a fairly heavy gauge wire for the main power leads to your secondary battery. You'll also want to make sure that the two batteries you use are fairly close in specification.

The main issue with a system like this is that the batteries can be come un-balanced, which can put strain on your existing electrical system. This is because your alternator is designed to vary the output depending on how much power is needed. If your existing battery is fully charged and you aren't running much in terms of accessories, the output is low. But when your battery is quite drained, the voltage can get as high as 14.5 volts in order to give the battery a full and complete charge.

When you have two batteries wired up like this, the alternator is measuring both of them as if they were a single battery. If you've been running the stereo all night and your house battery is quite drained, the alternator will try to charge it up quickly. But this can cause an overcharge condition with your main battery and drastically reduce it's expected life.

If the wires you use are too small, or the capacity of the second battery you install is drastically different from the one used to start your van, this can also cause huge problems.