I purchase a new 2017.5 Murano 3 days ago.

The first two days it worked fine - no problems.

This morning I leave for work ~ stop for caffeine ~ car won't start. Dash says "Key System Error" see owners manual

I ended up having it towed back to the dealership who keeps for 6 hours and then tells me that I can't have a cell phone charger plugged into the USB port when I try and start the car. He said I need to unplug the charging cable each time I try and start the car. Is this normal????? Please tell me that a car THIS SMART is able to let me leave a charging cable plugged in.

He said it's EF interference and the car thinks it's being stolen. Yet - it worked fine for 2.5 days. Any ideas?

Bummer. Electronics is difficult to troubleshoot.
I would drive over to the dealer's lot and try to reproduce the problem. That way you know the issue from personal experience. If the car locks up on you again you won't need a tow, you'll be right there.
Otherwise, if they were just guessing it could happen at any time. If you get a message on the screen, take a picture.

Once the dealership got it finished, they told me it was because I had a charging cord plugged in to one of the outlets without a phone attached to it. They said it puts off an RF interference that jams up the key fob. I don't know a charging cord puts off anything that would interfere with the signal from the fob, I thought the charging cords just put out POWER. Not only interfere with the signal sent from the fob to the car but then wipe the fob's programming all together???.... weird.

Your dealer is full of crap! I just bought a 2017.5 Murano SL 3 weeks ago and I leave my charging cable for my iPhone plugged in 24/7. I have started my car with and without the phone being plugged in without any issues at all. You need to keep pushing the issue and elevate it with Nissan. I also had a 2015 Rogue Sl/AWD and had no issues there doing the same thing.

Ken