We spent about two months in southern Utah this spring per the attached map. (The 'tent' icons represent our campsites).
Our first comment - - a general observation not related to Utah specifically - - is that the era of the "Itinerant Traveler/Camper" (at least in this 2020 Covid year) may be over. The crowds this summer have been enormous and, with the nearly universal adoption of "reservations only" campgrounds (virtually every state and federal facility in Utah, for example), the wandering camper is essentially locked-out of these campgrounds.
Your salvation may be your timing. In the past we have found that spring and fall camping represents a near perfect blend of fair weather and lowered crowds. But beware that the fall may still be in the "peak" season for southern Utah as that area is nearly intolerable mid-summer.
You did not mention one of our favorite southern Utah National Parks: Bryce. Don’t miss it. But here’s the real tip - - a hidden gem just a few miles southeast of Bryce (outside of Cannonville, Utah) - - Kodachrome State Park. There is a very nice campground in the Park and some not-too-difficult hikes, including the most popular (which we strongly recommend) “Angel’s Palace Trail”.
If you get down to St. Georges . . . there is a beautiful little state park nestled in the hugely picturesque red rocks, Snow Canyon State Park.
We can’t be of too much help on Zion as we arrived just as the park was opening, but before any of its campgrounds were open. We spent several nights at the Lamps Knoll BLM Campground west of Zion. It’s mostly a parking spot for some interesting climbs (technical and non-technical) into the rock formations behind it along with a number of tent camp sites ‘walking distance’ from the parking lot. But it was clear that campers such as ours were using it and, for us, the important feature was its ‘outhouse’. No water or electric though. By the way, we recommend the non-technical climb/hike - - good exercise and great views.
If you wander toward Corel Pink Sand Dunes (State Park) . . . mostly for people with ‘dune buggies’ . . . we found the nearby Ponderosa Grove BLM Campground much less hectic and modestly priced ($2.50 for seniors!). But no water. (This places you in striking distance of Kanab, Utah if that is on your itinerary.)
You also didn’t mention Capitol Reef National Park. There are several Forest Service campgrounds approaching Capitol Reef from the south on Route 12, but our favorite is the Single Tree Forest Service Campground that does have water and electric and some ‘first come, first served’ campsites.
Other worthy points of interest are Natural Bridges National Monument (a not too bad alternative to the very overcrowded Arches National Park). There are views available to the non-hiker as well as at least one doable hike (to an arch) that old-fossils such as ourselves can take. The campgrounds there were not open earlier this summer but suspect they are now.
And this will set you up for one of the ‘most interesting drives’ you can take, south from Natural Bridges on Utah Route 241 to the Moki Dugway. It’s very doable in a 26' RV, but the unguarded tight switchbacks and associated descent may take your breath away.
And while you're in the area, take-in Goosenecks State Park near Mexican Hat, Utah. Goosenecks is a very bland (minimal vegetation) but natural wonder along the San Juan River. Not nearly as famous (or crowded) as the Horseshoe Bend near Page, Arizona but, with its three ‘horsehoes’, arguably more spectacular. There's a State Park Campground right there - - and you probably don’t need a reservation!
Canyonlands has three major areas with two being accessible by car: 1) the eastern entry known as the Needles; and, 2) the northern entry (through Moab), known as Island in the Sky. Approaching the eastern “Needles” entry from the south along US 191, one of our favorite (we’ve been there several times) campgrounds is Devils Canyon Forest Service Campground. There are several BLM and/or Forest Service campgrounds as you approach the east Needles park boundary including Hamburger Rock BLM Campground (no H20), an immediate favorite due to its unique campsites in an odd collection of sandstone rocks. These BLM/FS campgrounds were utterly uncrowded when we were there, all first come, first served, but maybe the dearth of visitors was the ‘heat’ (at the end of May).
The north Island in the Sky approach to Canyonlands was problematic this year due to covid. Moab was one of the last counties to ‘open’ and allow visitors. And the Deadhorse State Park is worth the drive for its views, but as a reservation-only campground, beware. There are numerous really nice BLM campgrounds along the Colorado River as you drive northeast from Moab. These are in high demand, though. By the way, that drive northeast on Utah 128 along the Colorado is a ‘must drive’ in our opinion. Catch the right sun, and its overwhelming. A safer BLM campground that we didn’t stop at this year (although have stayed there in the past), which we drove through and concluded that there were plenty of sites available was Horsethief. This is along Utah 313 as you’re driving into Canyonlands from the north.
Let us know if we can add further insights.