Having been through Zions numerous times in cars/motorcycles, I would never pull a 31' trailer along Rte-9 through Zions National Park during peak season. Being March, the traffic/tourists/congestion will be down, and it is do-able. If you want to enjoy it, ditch the trailer and just take the truck through the park.
Easiest route is to go North on I-15 to Hwy-20 and over to Panguitch and back South to Bryce. An alternate would be to go along 389 to Fredonia then North through Kanab/ect to Bryce.
Hwy 20 will be the fastest/easiest for you.
389/89 will take longer but slightly more scenic.
Taking the 14 over Cedar Mountain (East out of Cedar City) is a huge climb followed by decent, but still better than pulling through Zions.
Taking the 9 through Zions will be the most scenic but the slowest route. It is the fastest route if you are on a motorcycle/small car though :B
