Hi kegman, and welcome to this Forum. There are some decent knowledgeable folk on here.
I have a collection of REAL 12v AGM's, 48v 1300ah, But now getting ready to change to VRLA. AGM's are very fussy with what they want.
I lost one a few years back, and opened it to see why. Yes, the cells contain no fluid but absorbent matts between the plates that act like sponges. Each of the 6 cells has a pressure release valve only, and has an secondary expansion area. So no physical way of getting electrolyte into the cell.
Reason for failure, over time the cell plates expand. They only need to expand 5mm upwards and the plate bars short out on the main buz bars of the battery, then the cell is effectively dead and the 12v battery will only charge to and hold at 10.6v to 11v.
Please be careful, eyes, skin etc, when messing with batteries even Glass matts types have the Acid present.