We have stayed at Poison Creek three times this summer. They have water, electric, AND sewer at all the sites, plus a very nice restroom/shower building. The sites are more open than at Ponderosa and the campground is right on the lake. There are two sections in the Ponderosa park now. The old section has tighter spots, but the newer RV part (which used to be the group campground) is quite nice.
I also second the idea of not driving over Highway 21 with a rig of that size. The Sun Valley area has a lot to see, and you would miss that by going 21.
They were paving the last of 21 near Lowman in 1992. Just about rattled the little car apart on the washboards every time we would drive to Boise from Challis when we were building our Stanley cabin. It's nice now that all we worry about on that highway are the avalanche closures in the wintertime.