There are a few campgrounds on the way to South Lake, I just hate paying, but the higher you can camp the night before, the better to acclimate. One trip I did camp down in the valley before the road climbed out of the low valley. I stayed the night along the creek where some dirt roads take off from the highway going to South and North Lakes, way below Aspen. My last trip, I could not see any place to stealth camp close to the TH, you'd have to hike in a ways then could suffer consequences of illegal camping/too early entry. Check out the CG's on the drive up for empty sites if you don't mind paying to stay in a CG. There is a small lodge on the way to South Lake, you have to cross by foot, a bridge to get to it. There used to be info over on Highsierratopix regarding it. On a side note, it is good to go armed with as much info as possible, but things do work out when only armed with limited info, go with the flow. On my vacation a month ago, I walked into Mammoth Visitor Center early afternoon and got my permit for the next days entry.