I agree with Cam that it would be worthwhile equalizing the batteries first to see what improvement might occur. I also agree that if/when you decide to replace them, it probably makes most sense to replace them in-kind, i.e., T-105s. They can produce enough for your needs.
However, I would caution you about drawing too many conclusions based on voltage decay alone. In particular, older batteries which have not been equalized and which have not been exercised (drawn down to 50% or so, and fully recharged a few times) may exhibit exactly the type of behavior you're seeing. What's instructive is that they have a great deal more energy left after reaching the 12.3 volts under a modest 10-20A load. See, for example, what this 225AH gelled bank did with only a 10A load. http://gallery.wdsg.com/Misc-Stuff-S...PhaseII?full=1
It reached 12.3V after about 180mins, then took another couple of hours to reach 12.0v and a lot longer to get to 11.5 volts under load. The curves for T-105s under load were very similar, and were uniformly at lower voltages (gelled and AGM batteries produce slightly higher voltages than do flooded batteries).
Another data point: I have six T-105s on my boat in a single bank which are 2 years old. They have been well treated, kept at 13.2V floating with a Victron charger while at dockside, and never discharged below 12.2v under load and never run dry. Yet with only a very modest load they will drop from 13.2v to 12.3 or 12.4 volts in just 20-30 minutes
. This is because they haven't been exercised since September and haven't been equalized. I expect (and hope) that as the spring cruising season gets underway and the batteries get exercised they will show much better "staying power". This has always been the case in the past (I've used T-105s for over a decade).
That said, at eight years your T-105s have most likely exceeded their useful life. And, yes, letting them run dry will do damage rather quickly. I'd therefore consider replacing them before you do any serious sailing.
Hope this helps a bit.