We've always had a good time at Buttercup, you just have to go over the hill past Plank Road. There is plenty of room, especially if you're arriving that early in the week.
The "Cul-de-Sac" can get very crowded, as does Midway. However, as said, that early in the week you can get a great spot on Midway and have easy access to both the roads and the dunes.
Take 5 South the 8 East, we usually fill up with water and last minute supplies in El Centro, then Buttercup is about 45 minutes East on 8. Take Gray's Well road and go right, where you'll actually be heading West along the freeway. Stop and get a pass if you don't already have one. Then stay on the road heading West. It comes up over a slight rise and after a few sweeping turns, Midway will be on the South (left) side of the road. Depending on how you want to orientate you rig, you can take the 1st, 2nd, or 3rd entry road and choose a spot.
Good Luck, have fun, and stay safe