6v battery fail

Hey Guys. Well I kept the batteries on a keeper all winter and they checked out good. I have installed them in my trailer and left them for a week checking the voltage (12.8v) at the start with a meter and hydrometer and left them for a week with the master turned off. They held a charge no problem. I then ran the furnace with it being 32f at night for 3 days and the batteries are still strong. I plugged in shore power and they topped up full. I have another theory about why they may have depleted. When I installed my inverter I mounted a 150 amp breaker on the back of the battery box. The terminals are exposed to weather. The breaker is waterproof so I didn't worry too much about it. But on the way home we hit extreme winter weather in Montana. (The batteries did not show signs of weakness until this point.) The entire trailer was encased in 2 inches of brown, salty ice. Including the terminals on the breaker. I am now thinking that this may have caused a ground to the chassy as I can't find anything else wrong. I am going to relocate the breaker inside the storage compartment to keep it dry even though the install instructions say it should be installed with in 12 inches of the battery terminal. Its unlikely I will ever use it to its full capacity of 1500 watts so I am thinking it should be fine. Most it will probably draw under normal conditions is 700 watts on the microwave.