Trust, but verify
If the dealer couldn't solve it, I probably wouldn't put a lot of stock in their assertions.
I'm trying to think through this logically. I would start by looking at the intermittent and total loss of power as a symptom and go from there. There may be a problem with the battery, ground, fuses or faulty connections (this last one sounds most probable).
If it were me, this is what I would do:
1. Check for stored trouble codes (most auto stores can do a basic check.)
2. Check the battery for proper voltage, clean terminals and proper connection.
3. Check both fuse blocks and examine your fuses for damage or corrosion. If you want to youtube it, check possible offending fuses with a multimeter for continuity. You can't always see if a fuse is bad by looking at it.
4. Check the battery negative cable to make sure it's grounded to the chasis.
5. Start looking at other grounds if all of the above fails.
Others, feel free to chime in. I'm not the electrical guru ;)