We usually stay in Camp Sherman when we go to Sisters. The private RV park is pretty much packed in sites. We like the Camp Sherman Campground. No hookups, no other amenities, either, but it is close enough to walk to the Camp Sherman store if you need something. It's right on the river. Lots of good things to see in that area, including the fish hatchery and the Headwaters of the Metolius.
Suttle Lake, just up the highway, is also a nice place to stay. There are three campgrounds right on the lake. No hookups or other amenities there, either, and not as quiet as Camp Sherman.