Fridge on 12V killing the batteries

oyly wrote:
Judging from the answers to questions I didn't ask, I did not make myself clear enough.

So, short and sweet:

The battery vendor says leaving the fridge in automatic mode will ruin the batteries, so I should switch it to propane mode. Is he correct?

The short answer is yes he is correct, you won't ruin another set of batteries that way. The long answer is it depends on how your alternator is wired and how many amps your alternator is actually putting back into your house batteries. For example I installed a 180 amp alternator when my stock 110 amp one failed trying to keep my reefer running while driving. Thinking this alternator would charge my batteries in a drive home was not even close. After a 5 hr drive home when my 2 6 CG volt batteries were at 50% SOC I still had to charge them over night with a 40 amp smart charger to bring them back to 100%. So some wiring charges the starting batteries first before the house batteries get charged. Unless you have an accurate ammeter you have no idea how many amps are actually getting into you batteries. What type of batteries do you have? How many total amp hrs? If you have the right set up at best I would cool the refer down over night on electric or propane and just use the 12v to maintain the refer temps while driving, then turn back to propane when the engine is off (auto mode).