Here's the official park website on the tunnel:
https://www.nps.gov/zion/planyourvisit/the-zion-mount-carmel-tunnel.htmYou aren't 'escorted' through it, you drive single file down the middle. Many RVers do it. If your RV is within the stated allowed dimensions then you'll have no problem except for the climb from Zion's valley floor up to the tunnel. Only you know how your RV would handle on a steep climb - or how you would do. :)
By the way, all national parks/monuments have an excellent web site such as this with lots of good information on things to do, camping, roads, etc. Check them out before you go.
Also, the best crossing for big RVs from I-15 to Hwy 89 is to take Hwy 20 at Circleville, UT. Do not take Hwy 14.
Before you get to Virgin, where are you driving from? There are options around that whole area. Where will you be staying to visit Zion?