I would pass on Whalers Rest it is mostly very tight. South Jetty is ok and at least in my opinion, on the Oregon coast you really don't need 50 amp as the need for air conditioning is almost non existent. The same can be said for Pacific City. Your biggest problem is arriving on Sunday July 2nd. By that time all the 4th of July holiday campers from both Portland and Eugene will already be there and you can be sure that the biggest and best sites will already be taken and the odds of finding a site big enough for your rig will be pretty slim. Almost everyone I know that is still working will take the Monday off and stretch this into a four day weekend.
If I was coming up from California I think I would be looking to stay further South say around Bandon at Bullards Beach state park until after the holiday. You would be far enough away from Portland and Eugene to avoid the large crowds and it is a great place to stay. Good luck