You're asking for camping spots "enroute to Yellowstone"; not at Yellowstone.
Use a mapping program such as Google Maps. Create your beginning & ending route and then put stops in every 300 or 400 miles (or whenever you plan to stop).
Then use something like https://www.campgroundreviews.com/
and enter the town where stopping and it will give you ideas at that town and also the surrounding area in case you want to stop earlier or later.
Enroute you don't need anything fancy perhaps even some WalMarts would work.
From your Profile it looks like you'll be starting from South Carolina. You'll have a good distance to drive so I wouldn't plan many, if any, siteseeing along the way. Save those days for Yellowstone and Grand Teton National Parks & the surrounding area.
(You might want to create a 'Signature' so that your RV information appears with every post you make. This helps those answering based on your information. Look under 'User CP' in 3rd line at top of this forum. I see you changed RVs.)