[Festus2]

I think that most Keystone products use a WFCO Power Converter. Not sure what your model is but mine is WF-8955AN-P. Its manual indicates that this is a 3-stage converter: Bulk/Charge, Absorption, and Float/Trickle. The output on the Float/Trickle mode is 13.2v.
It would be ok to leave your batteries in and have the unit plugged into shore power provided you keep a very close eye on the battery water level and make sure that it is topped up. A fully-charged battery can withstand cold temperatures.
If you can, I would suggest removing them for the winter and do the same that you did previously -- use a Battery Minder. Even when removed, you can still plug your unit into shore power. Be aware, however, that you won't be able to operate your slides with the batteries removed!