How long do you plan to be on the road? I've been to Zion about 4 or 5 times and each time it's been busier and crazier. The last time was a year ago October. The weather was still good and the park is still busy but not crazy busy. It may be a bit colder in March April but still probably comfortable.
We stayed at an RV Park in Kanab Ut. It's about a 30 to 40 minute drive to Zion. From Kanab, we also did day trips to Lake Powell, The North Rim of the Grand Canyon and Bryce. The North and South Rims of the Grand Canyon are two far apart to do both in one day. Both are very nice but the North Rim is not as touristy.
We camped at Kaibab Lake Campground when we visited the South Rim of the Grand Canyon. It's on Hwy 64 and closer to Williams AZ. Maybe an hour to the main entrance. If you start on the South Rim and then plan on going to Zion I'd take Hwy 64 through the park to Hwy 89 just south of Cameron AZ. From Cameron you probably have about a 2 or 3 hour drive to Kanab.
If you plan on doing Moab first, you have the option of taking the Northern or Southern route. If you have time, I'd take the Southern route to the Kanab/Grand Canyon area and visit Monument Valley if it's open. That's a must see.