From Niagra Falls, NY if you have your Passports, I'd suggest going into Canada and then into Michigan and work your way up to the Upper Peninsula of Michigan. Lots of beauty up there. Take the day trip by ferry to Mackinac Island at the 'big' Mighty Mac bridge separating lower and upper Michigan. No vehicles are allowed on the Island but they do rent bikes or surrey wagons.
Then take Hwy 2 across to Glacier Nat'l Park. From Glacier head south to Yellowstone and Grand Teton Nat'l Parks. They're next to each other but stay at both. Don't try to do both from one spot. In the Tetons we love Gros Ventre campground (our favorite) or Colter Bay campground (next to Colter Bay RV park) Neither one accepts reservations but both have 300 large sites and we've never had a problem getting a site.
Personally, I'd skip Yosemite and Custer but that's just us.
I'd then go south into Utah and definitely tour the big national parks in the southern part - Zion, Bryce, Capitol Reef, Arches, Canyonlands.
Then hit Grand Canyon - North Rim at the Utah/AZ border. If you like what you see and want more, work your way to Grand Canyon South Rim.
Head back home via Colorado Hwy 160. You could do Monument Valley, Mesa Verde Nat'l Park, the Durango and Pagosa Springs areas. You'll end up on I-25 around Pueblo, Colorado. From there, head home. Your days will have run out! Have a great trip.
We like getting off the interstates and seeing the real country of ours. We also like staying IN the national parks if you can. Each national park has a wonderful web site giving things to do, camping information, road issues, etc. Check them all out before you go.
Here is Glacier Nat'l Park's:
https://www.nps.gov/glac/index.htmGet yourself a good road atlas book and highlight a route you'd like to take. It will give you the whole picture and what to see along the way.
If you have time, order online through the publisher for the best price, the "Mountain Directory for Truckers and RVers". It will give you a heads up on the mountain highways. Don't be afraid to drive some of them. Just take it slow and enjoy the beauty!