You don't come up with a calculated plan of where to hide the bodies (oil tanks at your employer) without leaving the panic stage.
It's not like he put a sheet over them and walked out of the room. I would understand that if he just lost it and couldn't deal. But he loaded the bodies up, drove there, and used the tanks. At some point his mind made a rational plan of how/where to hide the bodies and thought it would work. Not sure if that happened before or after the murders, but he had a plan and executed.
It's also not like the first time LE asked about his kids he told the whole truth. He kept up that act for a couple of days.