You need to stay in a campground IN Yellowstone. You can't just overnight anywhere within the park or you WILL get kicked out. Don't plan to arrive in Yellowstone at sundown and expect to get a site. If you're planning for the non-reservable ones you need to be there early morning when folks are packing up to leave. Stay nearby the previous night and head for the campground early. If you can't plan that then definitely have a reservation to tour Yellowstone. Options are in West Yellowstone, Gardiner or Cody. Of course, you can go further away but you'll have a long drive full of traffic to get into the park and then you'll have many, many slow miles driving in the park every day.
Wal Marts aren't along the Oregon coast. Again, you need to arrive early. There are a few casino parking lots to stay. Some casinos are now starting to put in a campground so you'll need to pay for those.
Putting on miles in a RV is not the same as in a car. You're not going to be making fast time.
