There are several timeshares in St. George, but it's too far to do a day trip to Moab. (The fastest route is still a five hour drive each way.) You'd be money ahead to do as Karen suggests, and just plan a motel trip for the days you'd want to see Moab. There are so many great parks and rec areas in Utah, you could spend a couple of weeks and never see it all. Here's how we spent one week, and we didn't even get close to Moab:
We exchanged into a St. George timeshare for a week several years ago. We flew into Las Vegas, and drove the two hours to St. George. We spent the first two days there, seeing Zion National Park, which is a 45 minute drive away. Then we went up to Bryce Canyon National Park, where we stayed overnight in a Ruby's Inn motel right outside the Park. Next day we went over the Hogsback on Scenic Highway 12 to Capitol Reef National Park. We stayed overnight in a motel in Torrey, Utah. Next day we backtracked through the Escalante, saw Kodachrome Basin State Park, down through Panguitch, and further down through Kanab to the North Rim of Grand Canyon. We stayed overnight in a canyon-side log cabin near the lodge. Next day we headed back up through Zion again, and back to St. George, where we spent the last two nights of our week. When we got our rental car back to Las Vegas Airport on the way home, they said we'd put more than 1500 miles on the car during the week we'd had it. Lucky thing we had free mileage!
Dave