It's similar, but free to you.
Katherine, if you take your automobile club membership with you, you can pick up a book at AAA free of charge.
There is nothing the same in the US, as the Camps Book we have here... and which I use all over the country.
I have been a member of Good Sam in the past, but after being told at an RV park that if I wanted the discount they would have to charge me the full rate. I found I got better deals just talking with the staff, besides, the Good Sam places are amongst the most expensive.
Besides that, the book (and maps) from AAA are free - no money wasted on memberships which may not get you any benefit.
Definitely do the south first, though even by June Death Valley will be very hot.
I can't wait to read your trip report.
Lifey