Agree with the others - scratch the Olympic peninsula - not the best choice given the time restraints and probably not a good choice for your first RV trip either.
Start in Astoria - Fort Stevens State Park is nice - bit of a walk to beach however. Canon Beach has nice private RV Park - nice beach and touristy things. Beverly Beach near Newport is one of the better State Parks in the country, same goes for Harris Beach State Park down near Brookings. That time of the year you likely won't need reservations in Oregon .. but Calif is another matter.
