Marla is fine, you will fill up there anyway, Kulgera is a bit rough but call in and see the pub, which is first and last one in NT, depending on which way you are going.
Coober Pedy, we stayed at the Oasis on the way up, never again, and Opal Inn Hotel/Caravan park, very good.....meals and close to town, water station across the road, $1 for 100 litres from memory.
Like Sparkster said, we avoid Big 4, though it looked ok.
Desert Oaks rest area about 30k north of Erldunda is a good free camp and Lake Hart 30k north of Pimba is a great free camp as well.