It looked to me like the rear pod contained gasoline ("guzzleline"), the big tank contained fresh water, and there were buckets or containers on the side that were full of mothers' milk that Max splashed on his face.
When Max first encountered the tanker, they had a hose that just gushed fresh water for a long time. Max drank from that. I got the feeling that was from the enormous tank, and that was going to be used as payment to the gasoline manufacturer Furiosa was supposed to drive to.
Later, when Max had blood on his forehead, he splashed the mothers' milk from the separate small buckets / containers attached to the side of the tanker.
Several times there was mention of the "fuel pod" being towed behind the tanker.