In fact with coaches that have the dumb converter charger like the Paralax 7000 series the Winnebago manual says to disconnect the batteries with the BDS when the coach is connected to shore power for longer periods of time. That keeps the dumb charger from boiling the batteries dry.
If the switch is left so the batteries are connected you can always check the water once a month or so and prevent that but I suspect Winnebago was playing it safe with their recommendation .
You can see how they wire it below. When the BDS is open nothing is connected to the house batteries.