The slaves are crystal-driven right?
If so their timing is every bit as accurate as the RTC, so they can double buffer the readings and report the last buffer when asked. As long as the master doesn't take > 1sec to ask it works and is accurate.
Of course they would not all be in sync with each other, I don't know if that's a requirement.
That frees up a wire, not that you really need one, the comms can be done with a single 485 pair, it's just that the sync signal helps the slaves by freeing them from testing every byte on the wire.
record on a second by second basis but then to only squirt summaries every 10 seconds.
Then you can use the 1Hz signal as is to sync each slave, and after every falling edge on the 1Hz you poll the next slave. This way the existing signal does both jobs, slaves don't bother looking at the line until the sync signal drops, at this point they reset their internal timer and read the first byte. If it's not their address they carry on. If it is they respond.
That way you talk to each of the 5 slaves every 5 seconds. Naturally they have to be able to store that much data. How many readings does a slave take in a second?
Rob