If you're looking for dispersed camping near St. George then head to this BLM area overlooking Santa Clara. There's a lot of dispersed camping in the area, and some roads require a high-clearance vehicle to access. You can also find plenty of mountain biking trails in the area that are geared for intermediate-advanced riders.

Here are the coordinates for the campsite I stayed at on a recent trip to St. George: 37°07'38.8"N 113°40'30.1"W