When we went to Utah in the fall of 2013 and we had done a lot of research on which way to go to Zion. We considered coming from the west and south but in the end came through the tunnel and did the switchbacks. While our fiver is only 26 ft long by the time you count the tow vehicle and trailer we are a fairly long length also. The switchbacks were fairly insignificant in my opinion, if you went slowly and carefully there were no problems, perhaps a larger motor home with a toad would be different. The roads are in very good condition, unlike a lot of our switchback roads in British Columbia. The tunnel if you fit was not much concern except for the fact that you always wanted to look out of the side openings as you went through at the beautiful scenery.
As daunting as the road may seem on its reviews it is well worth traversing it for the scenery alone.
We came into and left Zion the same route and never felt uncomfortable at any time.
Maybe just good luck or whatever.