You are more likely to get the solitude you are seeking at Lerderderg if you avoid anywhere near O'Brien's Crossing and most of the north/north-west of the park.
In terms of driving into the park and car camping, as we normally park at the start of a track and walk in I am not sure of which (if any) tracks have car access. My recollection is that most (if not all) of the fire trails into Lerderderg are gated and locked. (It's not something I have paid much attention to, so could be wrong). The gates don't stop motorbikes, but they would stop a car. There is evidence on some of the tracks that 4WDs have created alternative pathways into some tracks. There are definitely places off Firth Road and Bluegum Track you could pull into and park/camp,some of which are quite large 'parking' areas, but I can't think of a track that you can drive into and be 'hidden' from passing vehicles. If you are willing to walk a short distance, there are ample places to go a little off track and find a quiet spot to camp. Next time I go out there, I'll have a look and let you know if I see any tracks you can legitimately drive into.
The major tracks are on the PV map
http://parkweb.vic.gov.au/__data/assets/pdf_file/0020/518240/Map-Lerderderg-State-Park.pdf. There are a few more tracks on the Meridian map (e.g., Hogan No to No 5 Tracks), but not all tracks. For example, Mill Track (?) halfway between the Cowen and Lerderderg tracks is not on either map. This track doesn't connect to any other track so is quiet, it is also a flat, short and easy walk to some old camping sites.