Winter Storage in Minnesota

Relying on the disconnect switch to fully disconnect the batteries is generally inadvisable as often there are at least a couple little loads that are not disconnected, such as the radio memory circuit or the CO or LP detector. Even a tiny load over the course of months adds up to a good bit of charge usage.

If the prevailing temperatures at the storage area will be below freezing, charging fully and disconnecting is fine. If the temperatures indoors are somewhat warmer on average, or there's concern about theft, it would probably be wiser to pull them and store them elsewhere where you can at least periodically give them a bit of a charge. (I'm assuming there's no power available at the storage place; if so, the best answer should be pretty obvious.)

Healthy batteries don't generally need to be constantly or periodically charged to survive the winter if they are fully disconnected. What I did in Fairbanks, Alaska (average nightly temp -55F, average daily temp -20F), was disconnect the ground leads. Next spring I put them on the charger and topped them off, they were fine. Did that over 3 winters there.

My battery compartment was lockable so I didn't even remove them from the RV battery compartment, just disconnected the grounds.

The acid in the electrolyte keeps them from freezing...if they are healthy batteries. Batteries on their last legs can freeze but if you can keep a charger on them at all times, even they will survive the winter. I used one of those small Schmacker float chargers to do that one year. Used that weak battery for another 2 years (chassis batt) when I went back south.