Almost all of the loss of electrolyte in a battery occurs when you get over 2.2V per cell or 13.2V. Assuming you don't have any loads that cause the converter to switch to charge mode very much you should be okay.
But as Drew said, you'll probably be fine with disconnecting them because the cold temperature reduces self-discharge to practically nothing.
Or maybe you can go with an option similar to how I take care of all of my unused or lightly used equipment over the winter. Top the batteries off once or twice over the winter with a portable charger.
