On the subject of measuring battery voltage, any solution involving potential dividers is going to involve a small standing current through the divider. The resistor values used here will be a trade-off between current drain and potential measurement accuracies due to having a high impedance source.
If this current is unacceptably high, there are high-voltage analog switches (The DG508 is an old example; there are probably more suitable parts nowadays) which can be used to select the input to the A-D converter.
When reading this, I was wondering about the possible application for a LTC1043 switched capacitor building block. It isn't cheap, but it would be ideal for sampling a voltage from points without a common ground. However, the maximum supply voltage is 18v........