Hi All - my husband and I are currently planning a road trip for next year (Sept) for 8 weeks starting in New York and ending up in Vegas. We have looked at out route and will be taking in Myrtle Beach, Atlanta, Nashville, Gracelands, New Orleans, Houston, San Antonio, Dallas, Phoenix, San Diego, LA, San Francisco, Sacramento, Yosemite Park, and then Vegas. We have never done a road trip before and would appreciate any help. We have looked at our route and picked out some RV parks and campsites but should be book them in advance or can we just turn up. We are from the UK and are very excited about this trip but feel a bit anxious as we dont really know what we are doing 
Go, you! That will be an epic trip and I’m so glad you’re doing it and taking your time. What a blessing.
just one comment at the moment: in between Atlanta and Nashville lies a lesser-known gem: Chattanooga, Tennessee. We have lived here for a little over seven years and I love it. It’s really very beautiful and there is so much to do here. Also, if the Civil War interests you at all, this is the epicenter and there is much to learn. The Ridge we live on the slopeside of, called Missionary Ridge, is dotted all over with monuments and signage describing troops/commanders from North and South, battles, situations, strategy, etc. And those signs, in addition to cannons, monuments, and other things, can be found all over the greater Chattanooga area.
If you are hikers, river, or train lovers, Chattanooga has you covered there, too, in spades. Here are a couple of videos to check out:
just 20 minutes from my house are some of the most curious and wonderful rock formations. You can see and experience them commercially at a wonderful place called rock city (seerockcity dot com) or, a bit further away, for about $5 or $10 at a state park called Cloudland Canyon. Rock City is the only place in town I have a season pass for! I love it. And if you’re traveling with a dog, per chance, they allow dogs! Mine loves it, as far as I can tell. 
Also, I have lived in Texas, Washington DC, Massachusetts, Connecticut, and Ohio; so if I think of anything more I can contribute to your planning, I’ll check back. Feel free to ask questions, too. I love traveling both here and abroad, love learning about the people and places wherever I go, and I love road trips!
, you may want to look at the mighty five national parks in southern Utah. From Vegas it’s not a very far drive to at least some of them. We were there in the spring and I can highly recommend it. While you’re in Vegas I would recommend going to the sphere if there’s something playing there that you would like. It is an unbelievable experience.
A little late to the party, but that’s a very interesting itinerary. You’ll be getting to California in Oct/Nov, which can mean cold temps and possibly snow at high altitudes, like Yosemite.
I’m a little surprised to see Sacramento on your list - it doesn’t pop in international travel circles often. That’s usually because it’s out of the way and doesn’t have much cachet.
8 weeks sounds like a lot, but you’ve got a lot on your list. Personally, I like 3 days for a big city or big NP. So I’d budget at least 2 weeks for San Diego-LA-SF-Yosemite-Vegas alone.
So I’d recommend looking at your route again and thinking about what cities/sights need more time, and what needs less. You’ll also want to plan in the travel time between places. SF to Vegas is an 8-hour drive no matter which route you take, and while you can have fun in that 8 hours, you won’t have time to see SF or Vegas on that day. Feel free to share your intended route for more detailed suggestions and/or reality checks. 
As to booking in advance - the nice thing is that you’re coming in the shoulder and off-season, so demand isn’t as crazy as usual. I’d also think a lot about what RV parks/campsites you’re looking at, since you’ve mostly listed cities as your destinations. RV parks and campsites are usually pretty far from the touristy parts of town, so you’ll just want to think about travel times in cities that may not have the best public transportation options.
Good luck!