That much amperage still flowing makes me think that you have some unknown DC load which is showing up on your ammeter as charging current.
NO cables, other than a battery temperature sensor, should goto the battery (-), that does not come directly from the battery side of the shunt.
I'd think 15 amps which never taper would have even 800AH of batteries heating. How much hotter are they than ambient?
My one flooded battery had one cell that was getting 10 degrees hotter on the bottom of the cell than the top or the other 5 cells, and this had me remove it from my rig.
But now it is on my workshop floor and in June will have been there for 3 years. mostly shallowly cycled.
Not sure whether just shooting the battery lid with IR gun inbetween cell dividers, will show the whole story with an AGM.