Fridge on 12V killing the batteries

First: My thought is that the battery vendor may have sold you older batteries that were not fully charged and your method of use finished them off. Those kinds of batteries are around every battery store, shop, parts department for whatever the reason and the seller decides they've been on the shelf to long and need to go. Good for them for taking care of you on the warranty. Running the fridge on 12 volts won't kill the battery as long as it gets properly recharged. The 12 volt heating element does deplete the battery and depending on what other work your alternator has to do will directly affect the amount of time it has to work on the batteries. Alternator charging ability has to be spread around to chassis batteries and house batteries and during travel it provides all the other 12 volt vehicle needs. Lights, radio, water pump, steps, interior lighting, and on and on so if it is a 110 amp alternator not all 110 amps are going to replenish the batteries. When starting out with depleted batteries it may not hurt to start the on board generator to help out with some of the battery recharge needs and take a little of the load off of the engine alternator.