Yeah I did that trip in July and lucked out when the Rock Springs supercharger was surprisingly up and running. Unfortunately for you it is no longer operational for some reason (It was never officially live on the Tesla website or in the Nav). Basically the only other places to charge on that route are in the KOAs which are in the same cities the superchargers will be in. So plan an overnight at the Rock Springs KOA and you should be able to make it.