Grand Canyon and the Canyons of Southeastern Utah.
Two very different places, both spectacular!
Grand Canyon: Wonderful huge canyon with lots and lots and lots of people. You get to walk to the overlooks, or trails along the rim, and view the canyon for as long as you wish. Get kind of old just sitting and looking w/o anything else to do. Then you get to go back to your RV or motel with little or no view. You can easily hike the rim trails, but going into the canyon you need to be pretty fit. Easy to hike down 500-1000 feet in elevation and not realize just how hard it is to get back up.
The Canyons of Southeastern Utah: If you dry camp or boondock you can park with great views w/o leaving the area of your RV. Wonderful sunrises, sunsets and night time sky viewing right from your RV. Some times of the year and some areas you can be separate by a 100 yards or more from your neighbor, not as easy as it used to be back in 1990's though. You can also drive for miles on paved and dirt roads with a new scenic view around each corner. Miles and miles of hiking trails that you don't have to go DOWN 500-1000 feet in elevation just to turn around and go back up.
Grand Canyon is well worth visiting for a week or two, but SE Utah has spectacular views and formations that will take many trips or months to see them all.