You do need to be aware that the Sisters Outdoor Quilt Show is the second Saturday in July, if that is your timeframe. There will be NO available space at the campgrounds in and near town. You can, however, usually find something out toward Camp Sherman or at Suttle Lake.
If you take Highway 20, there is a great county campground (River Bend) shortly before you get to Sweet Home. Very nice, private sites, clean restrooms and showers and easy on and off of Highway 20.
Also on that highway, if you see a sign that says "sunken grade", slow WAAAAY down.