As requested, I have drawn up a diagram of how to hook up a separate house battery in a van or any other vehicle:
My artistic ability is limited so I hope this is readable. If you have any questions feel free to ask.
The 10 and 16 on the wiring refers to the size, or gauge of the wiring. 10 gauge is larger and is the actual charging lead from the original starting battery. This heavier wire should be attached to the larger terminals on the solenoid. The 16 gauge wire (much smaller) is only used to turn the solenoid on and off and is hooked to the smaller terminals on the solenoid.
The 16 gauge wire from the solenoid can be hooked to any fuse or circuit that is turned on and off by the ignition switch and is only powered when the vehicle is running.
The negative terminals of the batteries and the negative terminal from the solenoid go to ground.
If you choose to use 6 volt golf cart batteries, they must be connected in series to make 12 volts
Multiple 12 volt batteries can be connected in parallel to increase the amount of power available.
Connections between the batteries should be made with standard battery cables. Only use 10 gauge wiring to provide a charging source to your house batteries. Do not try to run heavy draw items with 10 gauge wire.
If you plan on hooking up a 1,500 watt inverter be sure to use standard battery cables between the inverter and the house batteries as use of a microwave for example would cause the inverter to draw a lot of power from the house batteries.