If you are traveling on Highway 55, you could look at Lake Cascade State Park in Donnelly (south of McCall). The Poison Creek campground has full hookups and is right on the lake. Ponderosa State Park in McCall also has a campground with full hookups.
If you are headed through "the heart of Idaho", Redfish Lake or Stanley Lake campgrounds in Stanley are in one of the most spectacular areas of the country. The views of the Sawtooths are unsurpassed. This area is on Highway 75, just an hour or two (depending on how often you stop to take photos) from Sun Valley.