Elk Valley Casino on the south end of Cresent City allows free camping in their gravel lot just east of the main lot. The Casino is smack right at the start of Howland Hill Road. Howland Hill road is not recommended for RV's, but you could easily cycle it by doing an out and back or make a very long loop. It'd be dusty. After driving it for the first time this summer, I would definitely recommend it for the scenery.
Prairie Creek SP off of the Newton B Drury scenic parkway is not free, but it is very nicely located. There are some trails and double track open to mountain bikes nearby. You may want to research what trails are available for riding near there if only to stop for a ride.
Lucky 7 Casino just south of the Oregon border allows RV's to camp for free on the east side of their hotel.
In Oregon, The Mill Casino charges RV's to camp on their lot, but Three Rivers in Florence and Chinook Winds in Lincoln City do not.
It's tough to find much free camping along the Oregon coast. You could probably pull off on some of the larger pullouts if not signed no overnight camping, but many of the pullouts are signed as are the day use areas for no overnight parking. Driving inland may or may not find you much depending on the highway.
There's a very cool free campground inland northeast of Reedsport about 35 miles up the Lower Smith River Rd called Vincent Creek. Far up the North Fork of the Suislaw River out of Florence is a campground that would make a good base camp for a 40 mile loop ride involving that road, a gravel connector and Hwy 101.
There isn't much single track along the Oregon Coast. There are plenty of gravel roads waiting to be explored in the coast range.
There are some epic trails to be ridden inland including the McKenzie Trail, the Umpqua Trail, Waldo Lake, and others. There also a few freeride areas such as Black Rock.
