Some possibilities:
1) You mentioned that an afternoon of monitoring yielded a couple of transmissions on their listed frequency. Perhaps they are simply not very active. Many police departments these days have laptops in their patrol cars (often referred to as mobile data computers or terminals), and while this does not 100% replace radio communications, it can greatly reduce the traffic you hear on the air because they can look up license plates and other information without having to talk to the dispatcher.
2) Are you sure they aren't dispatched by the sheriff's office? I don't know much about Richland County, but it is increasingly common for agencies to outsource their dispatching to another agency to save money. This could be a recent change. One way to figure this out is to tune in the sheriff frequency and listen for street addresses that are inside your city.
3) They could very well have moved to MARCS. They would almost certainly retain VHF radios as a backup and to access inter-agency frequencies, so this might explain hearing occasional traffic on VHF and still seeing VHF antennas on cars.
Some non-technical ways you might figure out some of this information:
1) Search through news articles on local newspaper/media websites. If an agency changes dispatching arrangements or buys a bunch of MARCS radios, it's usually a big enough deal (at least around here) to warrant a news article.
2) Contact your local newspaper. Most media outlets monitor the local police on the scanner if possible (they have to get their news somewhere) and they might be willing to give you a tip about what system/frequency they're on. Try not to be annoying when you ask. A polite, well composed email would probably work best here.
3) You could always ask (nicely) a local cop if they've recently changed radios/frequencies/etc. Most police officers have no knowledge/interest in the details of their radio system, but might at least know if they switched to MARCS or something.
