Connect your jumper cables to the battery and take the other ends inside to your 12 volt breaker panel. First connect the negative lead to the negative terminal. If you have now got power from your batteries, the negative line is at fault. If not, disconnect the negative lead, and connect the positive lead to the positive side of your 12 volt service panel. If that gives you power from the batteries, then you have a positive line problem.
If neither single breaker cable works, connect both positive and negative and see if it works now.
