There are three ways to get from the south rim to Zion. The shortest is heading east on 64, turn north on 89 and then west on 89a and then head west on 9 at Kanab. 250 miles. Taking this route means you pay the $15 fee to drive through the tunnel at Zion.
The second route starts the same but at the Navajo Bridge you turn left on 89A and follow it to Hurricane then take 9 east to Zion. 280 miles. This is the route I always take so I don't have to pay the stupid tunnel fee.
If you wish to do the trip in two days there are some awesome campgrounds in Kaibab National Forest that you will pass on either of the first two routes. They are also the most mountainous, going up to 8000 feet.
The third route goes south from the south rim to Williams on 64 then west on I-40 to Kingman. Then north on 93 to Las Vegas. At Vegas go north on I-15 and turn east on 9 just north of St George. This is the least mountainous route. 430 miles and you get a chance to leave some of your money at the casinos to help pay my taxes. ;D >: (I am a Nevada resident and we don't have state taxes)
All three routes have their pluses and minuses. None of them are there any real problem towing a trailer.