Hi Jenklu, there are many options 3 hours far from BA, try to Gualeguaychu, or Colon, both in Entre Rios.
Closer than Sierra de la Ventana is Tandil, similar landscape but closer and with a much better road to it.
Pinamar Carilo don't have cool campings, but Ostende has a good one in front of the beach.
You can send me a PM if need more info.
Good luck in ur trip!