King Range - Mattole Campground, California

This blog post may be of some help -- scroll down to July 7 for photos of the campground area. But I gotta say, the road to the beach is very, very twisty. Don't know if it is suitable for a bigger rig.

Redwood trip, including Mattole Beach

Having been to the area, the gravel road is usually in bad condition.
I wouldn't take a new or big 5th wheel in there.

Google gave me quite a collection of images, including one of my Tiger at this campground. There are some fairly large Class A's in the photos. I am not sure how their size and maneuverability compare with your rig. The road in was moderately awful when I went (about 5 years ago) and I doubt that it has improved much.

link to Google images for Mattole Campground

Haven't been there......
Sounds like getting there could be interesting

This remote area is reached via a long winding drive down Lighthouse Road from the tiny town of Petrolia. Mattole Campground is at the beach and offers camping, picnic areas, and hiking opportunities.

Phone:
(707) 986-5400 for info

https://www.google.com/maps/place/Mattole+Campground/@40.2927082,-124.1039459,23059m/data=!3m1!1e3!4m5!3m4!1s0x0:0x90f3ef2e6c63d0f9!8m2!3d40.289116!4d-124.3562537

Do you mean the King Range Conservation Area near Whitethorn, CA?