Truk, by your title, do you have AGM house batteries?

It's my understanding that AGM (absorbed glass mat) batteries will receive no benefit by the "shaker". This equalization / disulfate charge will have no effect because the agm's have no liquid electrolyte (dry). The flooded or gel batteries are a different design and the disulfate mode "shakes off" the build up on the lead plates and the sulfate falls off the plates. The agm's are very tightly wrapped or matted plates with no electrolyte (liquid), so the equalization / disulfate has no effect. If fact, most three phase chargers mention that equalization may harm the agm battery (maximum charge 14.7v if my memory is correct).

If your voltage drops that rapidly, the batteries may be finished. But keep in mind, after a bulk charge, the batteries need to settle down for a few hours to give you an accurate voltage reading. The normal operating / constant drain parameters for an agm is no more than 1% drop in voltage over a period of one month, if no load or drain.