Heart Of the Hills has recommended RV length of 21' and only accepts up to max. 35-feet. This is directly from OP website:
Heart O'the Hills
"In old growth forest. Campfire programs in summer. 21' recommended RV length (some for up to 35'). Walk-in only if snowed in."
http://www.nps.gov/olym/planyourvisit/campgrounds.htmUnfortunately pretty much all the forest campgrounds in the OP are smaller/older. Pad sizes may show larger, but it's the turns and trees that get you.
I second the idea to look at Escapees. Another potential are the Clallam County Parks. They usually keep half their sites (or so) open to first-come-first-serve and they CAN take larger sizes. For example this park at Sequim:
http://www.clallam.net/Parks/Dungeness.htmlOr this one further west:
http://www.clallam.net/Parks/SaltCreek.html
