Its a awsome trip,done it many times,tons to do and see,its a windy twisty road so plan on takin your time,start in Washigton state at like long beach and head south,take your Quads the dunes are a blast on any machine,doesnt have to be a sport quad,the whole trip is one of my favs.
We just did this trip in October. We stayed at Fort Stevens State Park, Cape Blanco State Park and a KOA near Seattle while we were there. Both the State parks are very nice and were still quite busy for the time of year. We only have a small 24' motorhome but there sites are big enough to handle up to 50-60' units, I quite enjoyed having full power and water at both campgrounds for way less than we pay here for a plain no service site. We only stayed at a couple as we had my car to travel and sightsee with so it was much better. If you can go to the dunes, I decided not to but am kicking myself for not doing at least a short ride with a rental machine.
I would recomend staying in state parks along the oregon coast. You will have to book early to get a site in some of them. Yes I wish I had brought my quads but there will always be the next time.
We stayed in the horsefall state run campground on the Oregon coast at thE dunes just north of coos/Winchester bay when we went in June dunes are a blast although no where near as good as st Anthony very nice drive along the pacific coast hi way
We did it this past August. Stayed in State park campgrounds and no problems finding room without reservations other than 2 that were full up. Take the time to go to Northern California and the Redwoods and take the trip to see Crater Lake on the southern interior of Oregon. A spectacular site well worth it.