Hello Endofropie and welcome aboard!
Talking highway 9 through the Zion Canyon, correct? We took this tunnel 2 years ago. At that time the rangers 'manned' each end of the tunnel and allowed traffic one-way only: allowed folks to drive right down the center of the road, through the tunnel. No problems with height when you're strattling the center line. We sit 12' 7" including our ACs, however followed a series of bigger-diesel-rigs through, no problems.
Now this was in early May, before the heavy, seasonal RV traffic, so the wait was less than 15 minutes at each end.
I was more white-knuckled with that very last 8-10% down-grade coming back around on highway 59 as you drop back into Hurricane!