In Arizona maybe 20 miles or so south of the Utah border is the Virgin River Canyon Rec area. I've driven through it and it looked nice but I had my eyes and reservation at Snow Canyon SP.
Recreation.gov recreation area details - Virgin River Canyon Recreation Management Area - Recreation.gov
In Utah just across the border with Arizona near St George is the small town of Ivins and a super sweet state park called Snow Canyon State Park.
Killer geologic formations, Nice hiking trails and a great little CG with widely separated campsites with showers and a spaceport. A John Wayne movie was filmed there. I'd also recommend getting reservations there as the CG filled pretty quick after I arrived.
Snow Canyon State Park | Utah State Parks
That time I was there, there was some major construction on the interstate and traffic was backed up for miles so I took a quick detour off of I-15 onto Old Highway 91 through the Joshua Tree National Natural Landmark.Which I believe is surrounded by BLM. it' was a very scenic drive up and over the Beaver Dam Mountains and the Shivwitts Indian Reservation. It proved to be a nice detour into the back of the State Parks without having to deal with congested traffic and St.George.
Almost dead west from Snow Canyon and not too far out of the way is Gunlock State Park on a reservoir with boat dock and CG. I've never been but I had it marked on the map as a backup.
Gunlock State Park | Utah State Parks
Further north on I-15 just east of Cedar City on HWY 14 is the Cedar Breaks NM. 14 is a super awesome scenic drive that gets up up in elevation 9000+' with great scenic shots after passing through some cool canyons. There are a number of Dixie National Forest CG's along it's route.
I've never been to the Cedar Breaks myself only because each time I went by there, they still had some deep snow issues. One might expect that at 10,000'. So, call ahead. I hear it's worth it.
Cedar Breaks National Monument (U.S. National Park Service)
I'm sorry but, screw Zion NP unless you've never been there. A mad house of tourists unless you go in the off season but the only camping is in the south near Springdale. Some camping is 1st come, 1st serve but the rest is reservation through recreation.gov. Watchman CG in the park is full RV hookups. It helps if you're already in their system and be sure to ask for cancellations. There is a north entrance but you must go through the tunnel which has length width restrictions for larger busses and RV's. You have to pay an additional fee to pass through it but it is kinda cool. on the North side of the tunnel is the Zion NP overlook which is very spectacular especially in the winter with all the contrasting colors. There is a tiny parking lot though and easy to drive past with no way to turn around if pulling a trailer. If you do go to Zion NP, then to me the best hike is wading up the Virgin River Canyon. If you do, wear wet shoes and bringing your own hiking poles is a must! There are other things to do and see there but that's my personal favorite. You may get wet.
Zion National Park (U.S. National Park Service)
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