There's an Indian Reservation Casino CG just north of Pocatello off I-15. You'll have to check rvparkreviews for the details. It was cheap (not that cheap it's $27) but I don't recall how much. Once you start heading across the lava beds there really isn't much of anything except the occasional farm. The center of that area is the National Reactor Test site and you can't camp there.
Craters of the Moon has a CG ($7) but you'd have to divert south from Arco to get there.
I know there is a Walmart in Pocatello we went shopping there.