We traveled the L-C Trail in August/September 2017 from St. Louis to Cape Disappointment (with other things like the Eclipse sprinkled in) and listening to Undaunted Courage all the way. Highlights for us were the Winter Camp north of St. Louis, the Interpretive Center at Great Falls, Cape Disappointment and Fort Stevens. Also, any keelboat replica we could find "dad!, another keelboat?". Checking my records, we were in Cape Disappointment on September 11/12 and never had reservations for any of the spots. It was by no means empty but there was always something. I think the 16" Scamp
helps get us spots that other campers had to pass up. That was certainly the case at Heart o' the Hills in the OP. At Fort Disappointment we had a great spot steps from the pristine, empty beach. The only place we almost didn't get spot was late August at Kalaloch in the Olympic Peninsula. We stayed at private campgrounds about once a week for the pool/shower/power and those were never full. Oh, we happened upon a kite festival in Pacific Beach, WA around Labor Day. That was a nice surprise.