Canyonrover,
The picture above is the Surepower that SMB put in from 2004 to 2010?. Note the tab marked start signal. This was usually hooked up to the key switch start position. As daveb mentioned mentioned some people did not like it that way and may have wired it to a switch, in many cases a momentary switch. So if you have your key switch in the start position this wire would be high and force the solenoid closed. This would be be putting the batteries in parallel, so if you house battery was higher it would drop. If you have mystery switch, you should engage it and see what your batteries do, if the solenoid closes they should go to the same voltage. If you find that you have the wire attached to the start circuit,I am on the side of the fence that says remove it.
Also the diagram I posted in the beginning of this thread was a implementation that gets ride of the voltage monitoring part of the Surepower and just uses the solenoid. It would provide battery charging from the alternator when the engine was running, but it would not connect if plugged in, or charging the house battery with solar.