You don't have to miss the drive through the park on Hwy 9. Park the RV and drive it with your truck. I also agree that since you said you have a 29' trailer you could even drive it with your RV.
Get into low gear before you even begin a downhill; don't try to do it midway. Everyone will be driving slow in the tunnel section. You'll fit right in!
If you're 62+ make sure you have the Senior Pass for all these parks. It will get in in free and 1/2 off camping.
Here's some information on the tunnel area:
https://www.nps.gov/zion/frequently-asked-questions-about-the-zion-mt-carmel-highway-and-tunnel.htmI also suggest you look at the wonderful 'nps' web sites for all the national parks. They give lots of good information including things to do, special park alerts, road construction, camping, etc.
There are also many YouTube.com videos of special roads.
https://www.youtube.com/watch?v=jZmoVAfKaMg