carl1949, you CAN go through Zion on Route 9 with an RV, but you will have to pay the park entry fee (good for re-entry for 7 days) and pay an "escort" fee of $15 to go through the 1.1 mile-long Zion Mt. Carmel Tunnel if you are over 7' 10" wide and/or over 11' 4" high. However, you must be less than 13' 1" high. Check out
this link. They call it an escort fee, but they really just stop opposing traffic so you can drive down the middle of the tunnel where it is taller. The fee is good for 2 trips within 7 days. In addition to the tunnel, there are also a few switch-backs and tight turns along the way; but we saw several large, diesel motor homes and a couple of 5th wheels that had gone through or that were waiting to go through when we were there this July.
We chose not to go through the park with our RV and stayed in Glendale, UT because we also wanted to see Bryce, Cedar Breaks, and Escalante Grand Staircase. Glendale is kind of in the middle of all of these parks.
We were traveling from west to east instead of east to west as you will be doing; but we came up I-15, across UT Rt. 59/AZ Rt. 389, and through Kanab. After seeing the area parks, we continued east through Page and through Monument Valley with a stop along the way at the North Rim.
The only other alternative is the northern route up I-15 and across I-70. This is quite far out of the way. The southern route is a little out of the way, but very scenic.
If you're trying to make time, I would recommend the direct route through the park as long as you are comfortable in being able to control your RV and keep it in the center of the tunnel and maneuver the turns. If possible, hit the tunnel as early as possible in the morning to avoid traffic delays. If you are uncomfortable with the tunnel or the turns or if you are staying in the area anyway and you're not in a big hurry, then I recommend the southern route.