Total Electrical Failure while driving.

While driving my 2008 Ford Escape Hybrid last night I experienced a total
electrical failure. The car had been operating normally all day , I was driving
home 35 mph at night with A/C on when everything went completely dark. I
mean everything, lights, engine , dash gauges failed in position, no dome
lights, no emergency flashers , nothing. Got it towed to my shop . I tried
another 12 volt battery , still nothing. Checked the chassis ground at the 12
volt battery , still nothing. Unfortunately I have no schematics  and the
positive cable from the battery disappears into a bundle.
Is there one relay or contactor that would cause this condition ? Bad ignition
switch ? This would have been a dangerous situation on the interstate.
Thanks