We stayed at Red Rock RV Park in Island Park, ID a few years ago when we visited Yellowstone N.P. It was about a 20 mile drive up to the West Entrance, but we always enjoyed the drive.
That area of Idaho is worth a visit in its own right. We enjoyed Johnny Sack's cabin and that entire area. There was also a youth rodeo about a half mile down the road from the campground.
That's the only campground in Idaho where we have stayed.
http://dwilliamson.com/Yellowstone/Yel7_jpg.html