Best route to Zion National Park from Mesa AZ for class A?

I-17 to Flag then 89 north through Page, to Zion. We do it with our toy hauler, easy. We're just as high as you and there was nothing in our way. I wouldn't drive it into Zion - take the toad (easier to pull over and see the sights) - but the big tour buses go through Zion all the time.

What is the 11'H tunnel you speak of?

60W to 101N... 17 North to either Carefree Hwy, or 303 to Lake Pleasant Rd to Carefree Hwy. 93 N through Wickenburg to 40 to 93 bypass through Hoover Dam. Then 15 North from Vegas. That will bring you in through Virgin, and no tunnel to deal with. Easy peasy.

Parker is far out of the way, and twisty, slow roads.

Have a fun trip. Maybe see ya there!

If it were me I would go west to Parker then up through Laughlin north to Las Vegas then North to St George!
There may be limited Campsites at Zion ,St George is a short drive in your Tow car!

“What are the size limits for an oversized vehicle?
A. Any vehicle that is 11 feet 4 inches high or higher and 7 feet 10 inches wide or wider needs the one-way traffic control service. Any vehicle 13 feet high or higher cannot pass through the tunnel.
Length restrictions are 40 feet for a single vehicle and 50 feet for any vehicle combination.}

The best route is the most scenic route...for me.