You are going to roast. Phantom is already 90s during the day and 60s at night, only going up. The coolest night I remember having was at Lee's on the June into July trip I was on, and then it was only up from there. If you are definitely going to take that bag I would buy a set of sheets from walmart to pack. One for on top of the pad and one for over. keep the bag at your feet unzipped as a comforter to pull up at 4 am when you get chilled. I think sleeping on top of it would be hot too.
Honestly, I would leave the fancy new bag at home and take something cheaper and lighter. Sand will infiltrate everything you take on the trip. My wife and I on desert river trips just push the two pads together, fitted sheet over both, flat sheet, and then a light down comforter if needed. You won't need to be bundled up into a bag. 60s might feel cool after you roast in the sun all day, but I would still say a 15 deg bag is overkill.
have an awesome trip! it's a whole other world down there.