Most of the National Parks you mentioned (except Death Valley in August) book up in seconds about 6 months in advance. Add the 40 foot RV and you are in for a very hard time.
I wouldn't go to Death Valley in the summer if YOU paid for the trip. Some love it. I get it, but no thanks. I was there a few months ago. Lovely place during the winter time. We barely got a reservation in winter. The "good news" is you can book a spot the day of at Death Valley in August.
As was said, your best options will be private campgrounds that aren't too close to your major destinations and/or far away from the coast. Basically, start planning for next summer. I wish I could help more. My apologies if we seem like Debbie Downers. We just don't want you to drive thousands of miles for nothing.