When Central County E911 dispatched departments were still dispatched on vhf and not SLATER they were split North (North Dispatch)/Central (Central Dispatch)/South (South Dispatch). Those departments retained that scheme HOWEVER on SLATER they are then divided to either North (North Dispatch, North Main, TAC 10-19) or South (South Dispatch, South Main, TAC 20-29). The dividing line for departments is roughly Page Avenue.
Monarch for example get's toneouts on VHF via Central Dispatch 154.220 but responds and operates on the South Talkgroups. Mehlville for example get's dispatched with toneouts on VHF via South Dispatch 154.160 and responses and operates on the South Talkgroups. They did this so department's didn't have to reprogram their station alerting. There is no paging capability currently on SLATER so if the stations wanted to retain their station alerting via radio they have to do it with the old vhf frequencies.
The only thing currently simulcasting on vhf and SLATER is Dispatch. All traffic is on SLATER. So if you wanted to monitor the fire departments but also hear their tones then you need to also monitor 154.325 (north dispatch), 154.220 (central dispatch), and 154.160 (South Dispatch).
Enjoy the tones while you can.