I'm pretty meh on Sand Flats. Lots of rednecks flying by in ATVs and quads at all hours. It's okay if you want to camp on dirt and have to be right next to town, I guess. There are so many great BLM possibilities near Moab. Don't waste your time pre-scouting a grid coordinate on Google Earth. Find a general area with good options (see above) and drive there. You'll find what you want easily.
I don't think any truly awesome campsites are going to see their grids posted here. There's a reason that they're awesome.