It's been a very long time since I've been there so consider that.........
I doubt the lake sites are "crowded" at any time, after mid-August there are probably few people on the lake although the campground itself may still have quite a few people. By September it's probably very quiet. I stayed at a very nice island site (around the "bend" from the big cliff with the pictographs), other than being close to the site where there was a "famous" fatal bear attack (many years after I was there) it was not very "primitive" (picnic table).
The first time I was in the park it was to launch for a trip north on the river to Mattice, we stayed in the campground the night of arrival, I remember the baited bear trap set up just down the road from our site!
You might run into some motorboats on the lake but as usual these boats don't stray too far from the campground and probably rarely use the "interior" sites except for a lunch stop.
The lake can be windy and the nicest parts of the lake, south and west arm, are quite a distance from the campground. You might want to be prepared to stay out an extra day. I remember at the time thinking it would be a nice place for a kayak.
