So.
With another r/r I am getting a good voltage range directly from the 'hot' lead charging the battery - but voltage across the battery terminals is not changing appreciably (the battery was not charged fully, and is currently resting at about 12.3V and on a charger). Should note that it did not occur to me to check the r/r that was on there previously in that fashion before i put this one on.
I will leave the battery on a tender overnight and check the voltage before leaving for work tomorrow - then check resting voltage at work, and if it has not fallen, check to see if I am seeing the charging voltage across its terminals.
If it fails either test I will walk across the street and swap out the battery. As it's an AGM I am unable to check for bad cells with a hydrometer.