Since you've got no power whatsoever in the vehicle when the engine shuts off, the first thing I'd examine, are the battery cables. Follow them from the battery itself all the way to the ends, to check for anything that might be shorting or touching.
In this case, we also need to assess the condition of the battery when this occurs. To diagnose further, bring a voltmeter with you when you drive, and if it should happen again, use the voltmeter to measure the voltage at the battery when the system is dead. Then, remove the positive battery cable, and measure the voltage again.
If you get a very low voltage reading with the cables connected, but it comes back when they are disconnected, you have a short in the system. If it stays low either way, the short is inside the battery. If the voltage is good when tested, you may have an intermittent connection at a main fuse.
Give that a try, and I can help you work it from there if needed be. Good luck!