Reddit mentions of Camping America's Guide to Free and Low-Cost Campgrounds: Includes Campgrounds $12 and Under in the United States (Don Wright's Guide to Free Campgrounds)

My girlfriend and I are in the middle of a year-long RV trip (with, apparently, a six month gap while we winter in Savannah).

I think your first step has to be deciding what it is you want to see. Make a wishlist of all the places you want to visit. You won't hit them all (and you shouldn't even try), but having a list helps you plan your route. (I used and use Google maps to plot where we've been and the places we'd like to go.)

Once you've plotted all the things you'd like to do on a map, you can start thinking about rough routes. (I'd encourage you though to keep your plans fluid. One of the best parts about traveling by RV is that you can change your itinerary on the fly.)

Not sure why you'd want to rent an RV in North Dakota (or anywhere) else and then drive it to the Northwest. Start in the place you want to explore, otherwise you'll waste days getting to your destination.

Finding places to park isn't tough. There are tons of RV parks, obviously. Plus, the West is filled with public lands where you can park. We spent $15 to buy this catalog of low-cost campgrounds and we've used it tons. Highly recommended.

The best way to figure out how much space you need and what kind of RV is to get hands-on experience. Go to an RV show. Go to an RV dealership. (Or several.) Browse around. See what's different about different models. If you can, borrow one from a friend and spend the night somewhere near your home. I have no idea what kinds of motorhomes are available to rent (there are links to rental sites in the sidebar to this subreddit), but we were glad we did so much hands-on experimentation before we bought.

Most of all, have fun!