Utah National Parks and Monuments.

TenOC wrote:
Zion -- all parking is full by 9:00 AM

More important, if you didn't sign up for shuttle tickets ahead of time, it doesn't matter when you get there if you want to go up the main canyon.

You can still walk in but when we were returning from the shuttle stop by the Narrows, a couple had lost their ticket and the staff was going to make them walk the 8 miles back to the entrance. Our shuttle pulled out before we saw the conclusion but the staff seemed pretty firm about it.

We've been to all of them multiple times over 30 years and there are many comprehensive threads about visiting them. In April expect snow, Zion and Arches may only be overcrowded instead of packed like a sardine can.
My suggestion is to visit parts of the parks that aren't as crowded, get there very early and hike less popular trails. Go to less popular parks, Bryce, Capital Reef, the North Rim and Canyonlands. The are lots of state parks too.

Many people on multi-week trips include parks and sites in Northern AZ as well along with southern Utah parks. Specifically Grand Canyon NP, Sedona, Page/Lake Powell, Monument Valley.

Right now, due to covid, the Navajo Nation is closed. This means, several sites including Monument Valley park, Antelope Slot Canyon in Page, the road to the east entrance of GCNP South Rim are closed. Who knows what the situation will be in April.

As noted above, study the Zion NP website for the Zion entrance tunnel and covid restrictions related to shuttles and reservations. Probably best to try to stay at the campground inside the park or in Springdale if those shuttles are running. I would check all Nat'l Park websites for covid restrictions. They could change at any time.

Zion -- all parking is full by 9:00 AM

We hit them this fall.

Make sure to check the entry requirements. Many had daily limits and you needed a reservation (particularly Zion). Some had none. It seemed to be a moving target.

Don't miss Dinosaur NM.

I've been to most of the places around Moab. Arches is awesome but be ready to walk a lot to some of them, like Delicate Arch. Canyonlands, north and south is also near Moab.
Go to Bryce and there's a great 2.5 mile walk between Sunset and Sunrise points, that takes you thru Wall Street. I think it's N-E of Bryce on hiway 12 is the Escalante Grand Staircase... a really interesting drive.
Zion is also a good walk, especially if you want to go beyond the end where the stream narrows.
The drive thru Monument Valley is terrific. I think it's Gouldings Trading Post was a good place to stop and take the Pink Jeep tour into the Navajo lands. BE SURE it's the Navajo guided tour because the Navajo guides can take you deep into the canyons where you aren't allowed to go by yourselves.
That's just a few places I've been and there's still a LOT of Utah I want to see. Craig
^^^ Oh yeah, Trail Ridge Road required reservations last year but not this year. We're camping at the bottom of Big Thompson Canyon in July, Riverview RV park. The wifey has never seen RMNP.
FWIW, Sylvan Dale Guest Ranch is awesome but they don't have RV spots anymore.

We are doing the same thing only hitting the 4 national parks in Colorado We leave April 17th

We hit all 5 parks in Utah a few years ago we used the following parks

Zion river resort Virgin Utah
Ruby's Inn Bryce Utah
Thousand Lakes RV Park Torrey Utah

We were there on April 17th thru May 6th 2015 and we had cold weather to include snow - you might want to head south and keep warm then head north to the parks in Utah

Can not recommend the park we used in Moab

Have fun

In planning my Colorado trip I find some sold out dates so reserve ahead