wanting to go to Zion, Arches, etc how would the last two weeks of April be for visiting?

Pretty amazing.

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The end of April is typically great weather in Southern Utah.
That will be a very popular time of year for Zion, so be sure to make reservations well in advance.

Definitely plan ahead. Went in October and it was amazing.

Epic

It would be great. I've gone that time of the year a number of times.

The only negative would be if you were planning on hiking the Narrows in Zion.

The water level and flow due to spring melt might (probably would) preclude you from doing this.

Be advised to take warm clothing if you're going to Bryce Canyon - which I would highly recommend.
It's at between 8,000 and 9,000 feet elevation and will still be pretty cold especially at night and early morning. Some of its trails might still be icy also.

Also the North Rim of the Grand Canyon might still be closed. Usually opens in early May.

we are headed across Utah in late May 2019 - renting an RV

what do you mean by make reservations now? hotels or are you referring to other activities we should plan for?

appreciate the advice!

Hotel reservations and campsite reservations. For tent camping, the sites within the national parks open for reservations 6 months in advance and fill quickly. I would look into reservations at RV campsites. I typically do tent camping, so I am not as familiar with RV reservations.

I'm in the early stages of planning a road trip from Vegas through Utah & N. Arizona.
My main concern are crowds as the earliest I could go would be the first week of June.
How bad will it be?

you still planning this trip? we leave next Saturday and go from Phoenix though Utah and over to Las Vegas

I'd like to make a similar trip one day. If you don't mind, let me know how it goes.

FYI - this is my high-level plan - I'm building this out with depth including miles, parking, hikes, etc - we're renting a 30 ft RV - some of these are just drive-by's but I want to see everything the area has to offer, even if we don't stay but 30 minutes

12 days/11 nights 1,455 Miles

Phoenix 0 nights Camelback Mountain
Sedona 1 night Sedona, Slide Rock, Grand Canyon, Horseshoe Bend
Page - 1 night Lake Powell, Antelope Canyon, Monument Valley
Monument Valley 1 night Arches National Park
Moab 1 night Canyonlands NP, Dead Horse State Park
Moab 1 night Goblin Valley, Little Wild Horse Canyon, Capitol Reef
Torrey 1 night - Capitol Reef, Boulder, Escalante
Bryce Canyon 1 night Bryce Canyon, Grand Staircase, Willis Creek Slot, Kodachrome
Mount Carmel 2 nights Zion, Red Canyon Slot, Coral Pink Sand Dunes
Las Vegas 2 nights Bellagio