6 volt versus 12 volt HELP

Thanks for the advice. When boondocking I charge them with a Yamaha ef2000is. At night we are very consertive with the lights ( headlamps mostly) and the heater is the only thing on. I can't find anything else drawing significant power. We have never had problems with these batteries before this weekend. This time 3 nights in a row we went to bed with the meter in the camper saying they were full and then lost power sometime during the night. I thought they might be low on water but checked and they were full. When the camper is stored is has constant shore power.