What type of RV are you renting and will you have a toad or tow vehicle? I agree just do the coast portion and take your time. In Crescent City CA we stayed at the KOA. If you have another vehicle other than the RV there is a great scenic drive in Crescent City through the Redwoods. Howland Hills Road. You cannot take the RV on that road. http://www.nps.gov/redw/planyourvisit/drives.htm#CP_JUMP_394252
If you can't do the scenic drive, you can go on down to Redcrest CA and stay at Ancient Redwoods RV Park on Avenue of the Giants. You can easily walk to see redwood trees from there.
Don't miss Tillamook Cheese and Ice Cream Factory south of Astoria. Nice cafe for lunch.
