As Spitfire said, both of these systems would be programmed as MOT 800/900 TSYS types. The Palmetto 800 system would need to have the custom table information set for a standard 800MHz rebanded system. Since the Horry County system doesn't use any rebanded frequencies, it's not necessary to set the custom tables for that TSYS object. It also shouldn't hurt anything if you do set the custom tables - it should work either way.
For Palmetto 800, you'll want to program site 38 Conway, 39 Georgetown or site 61 North Myrtle Beach depending on what area of the Myrtle Beach areas you'll be visiting. You'll hear state agencies such as SCHP, SC DOT Response Vehicles and Medical Helicopters on Palmetto 800 in the MB area, along with some utilities. All of the Horry County and Myrtle Beach public safety agencies operate on the Horry County system.