The west is BIG and you’ll only be able to scratch the surface even with a month. I too would choose Yellowstone and the Tetons over Grand Canyon. Mostly because it’s spectacular country and there’s more variety to see. But the Canyon is impressive in its own right and it’s close to the many national parks in Utah. And that far south you could also hit southern CO and check out Mesa Verde and the Durango-Silverton train ride. It’s getting a little expensive but it’s quite a trip. If you do that take the open cars. You’ll come back with a faint coating of coal dust but you’ll have experienced it rather than watched it through a window.
You’ll just have to plan a few loops and see what you can cover with them while allowing enough time to enjoy it without excessive driving. Most kids are interested in dinosaurs and the park is in NE UT and NW CO so it’s more conducive to a northern loop. We took our boys there when they were young and the youngest especially was fascinated with it. Friends used to joke they had a masters degree in dinosaurs after talking to him.
Recognizing that tastes differ, I think places like Cedar Point and Disneyland aren’t even an also ran. Overpriced, overcrowded and with little to offer other than rides and junk food. But then, kids do like that stuff. We traveled all over the western national parks with our two granddaughters for years. I think my wife wanted a trip to DL so she asked my youngest granddaughter, who was then 14 or 15, if she wanted to go there, knowing I’d do it if she did. It backfired, my GD said no, not if it means we can’t go camping. I was really proud of her. :B
If making that trip in summer remember that southern UT will be pretty hot during the day, though it does cools off at night. Zion’s one of my favorites and it’s tolerable in the shade, Bryce is 8000’ and cooler. I don’t know what your RV is or how you camp, but there’s a lot of public land in the west that you can dry camp on and not be hindered with reservations if that’s your choice. Onece you decide where you want to go and how people on the forum can give you some good campsite options. One certainty, you will be making more trips.