Even a single stage converter, operating properly with an output of 13.6V or thereabouts, won't boil batteries dry too quickly. Just check and add water as needed (maybe every month, at least until you get an idea for how often it's required).
With a PD 9600 series converter, I top up the battery water once a year, whether they need it or not.
In areas where it's sub-freezing, you can also just fully disconnect the batteries for the winter (after charging them fully) and leave them and they'll be fine...or if they aren't, they weren't fine to begin with. Storing them in a warmer place requires periodic or float charging as the rate of self-discharge is temperature dependent..
