Military Vehicle Repair: voltmeter., deep cycle battery, truck batteries

Question
I recently purchased a m1009, which I just converted to run on vegetable oil, but I am having a problem with my batteries staying charged. It seems that my rear battery wont stay charged. The volt meter was disconnected when I bought the truck, so I hooked it back up the way I thought it should be. There were only two wires to hook up to it, but there were three posts on the back. The posts said "IGN", "GND", and something else that I forget. The two wires were black and orange (I think). I added a wire to the ground post (I grounded it to the frame), but after reading a post on this website, it seems that I may have been to cause of all my battery problems.
  My voltmeter is only on now when the car is off. It says I am slightly low (in the yellow). Should the voltmeter be on when the car is on or off. How should it be properly hooked up?
  Also, if there a way to make it so one alternator charges both batteries, so that I can use the second battery to charge a deep cycle house battery to use when I'm camping?
Thanks, Joe

Answer
The volt meter should only be registering when the ignition is on. There is a relay that should operate it, and sounds like you bypassed it.

Connect your deep cycle battery terminals to either one of the truck batteries...positive on one battery to positive on the deep cycle, and neg to neg. Just don't connect anything else to the deep cycle at the same time if you use the rear battery.
If you connect to the front battery, you can have a 12 volt item connected, and use it at the same time.

Van