We live in the Boise area and travel the mountain road a lot. The best way from Boise to Stanley, is Hwy 55 to Banks. Take the Banks Lowman Road to Lowman, then north on Hwy 21 all the way to Stanley. Hwy 21 from Idaho City to Lowman then Stanley is way too winding for any RV driving. Then from Stanley you can connect to 75 and head South through Ketchum and Sun Valley to Twin Falls, or another way would be to head north on 75 to Challis, then through the Peaks to Craters scenic Byway to Arco, then drop down into the Twin Falls area.
Off the Banks Lowman Road are several forest service campgrounds, but only one or two that you could fit in - Hot Springs right off the road on the left (in the direction you're going), or Pine Flats on the right. The entrance to Pine Flats is pretty tight, but I've seen some big rigs in there. Near the Lowman fire district are boondocking sites along the South Fork of the Payette River. Stanley has some hookup sites, one is at the Valley Creek Lodge, which is basically just a few sites between a couple of lodges, but the view is beautiful. Also, there is the Mountain Village Resort at the end of town overlooking the Salmon River. All pull through sites. Further down, if you're going to go through Challs, is Torrey's Burnt Creek Inn. Cabins and an RV Park with full hookups right on the Salmon River. Worth driving down to stay there even if you're going to return and go through Sun Valley. I know the owners. I sell my art work in their little gift shop. Great location. A lot of boondocking sites along Hwy 75 between Galena Summit and Ketchum.
2011 Bighorn 3055RL, 2011 F350 DRW 6.7L 4x4 Diesel Lariat and Hensley TrailerSaver BD3, 1992 Jeep ZJ and 1978 Coleman Concord Pop-Up for remote camping
Dave & Renee plus (Champ, Molly, Paris, Missy, and Maggie in spirit), Mica, Mabel, and Melton