Its not uncommon for the batteries to fully discharge during winter storage, especially if there is some parasitic loads on them. Turning the contactor OFF does not necessarily mean you have removed all loads, especially if you had the dealer install any aftermarket goodies.
Typically, when in storage, you should keep the RV plugged in over the winter if you can. I plug my coach in 2 weeks then unplug 2 weeks, just to keep the batteries charged.
If you have no source of shorepower over the winter, you should start the generator once a month and let it run to charge the batteries. Depending on the coach, you may also be able to charge the coach batteries with the engine alternator.
You should be running the engine and generator once a month or so anyway during storage.
I have really never heard of acid coming out the top of a battery. If it is cracked, it will run out the sides, but not the top.
If a battery is fully charged, the concentration of sulfuric acid suspended in the electrolyte will be the greatest. The concentration of sulfuric acid prevents the battery from freezing.
However, as the battery disharges, that acid migrates to the battery's plates, and the electrolyte almost becomes water. At that concentration, the electolyte can freeze and become ice, and burst the battery.
So if you store a battery in the winter and if temps get to freezing, and you allow the battery to become fully discharged, it can potentially burst.
The only thing that works on a RV is the owner...