Spent two nights at the mouth of the Toutle. It was tolerable at best. Found a spot near the river which was a mix of sand and ash. Beat camping right in the rocks. Lots of people around chasing silvers. 
 Drifted I-5 to Mouth without success. The bottom two miles or so are frog water, mostly sledders backtrolling plugs, or backbouncing bait. Saw few fish caught, but since the rain? At the time you could walk across the Toutle.
 Blue Creek is not too bad if you get one of the three spots in the woods. Bring a trash bag. (or two, well maybe even three!)
 I have camped on the Cowlitz itself in the past. We camped right on the bank right above the log cabin hole. I may have been trespassing, but we were in late and out early, and leaving no trace, we weren't too concerned. This summer we saw tenters below the clay bank below Blue Creek. They survived the BIG Thunderstorm in July. Didn't look like a real good spot (and you sure can't get there in your truck anyhow 

)
 Another nice spot is the Barrier Dam Campground. Reasonable rates I suspect. Close to town (Salkum) and the fine dining establishment, the Brown Shack.
 I sure hope someone has some better information for you than this.
Guts