if it's what he said, most likely the house battery.
Very common for them to be delivered dead. Scenario....the coach is sitting on the deal lot. Someone turns on a light..... never turns it off. The next day, the battery is dead. These wet cell batteries if deeply discharged like that are damaged. Once or twice full cold dead and they will no longer hold much charge.
the first thing I did tour coach was to add mechanical disconnect switches on the posts of both batteries (the house and the chassis). When i store it, even if just for a day, the batteries get disconnected.
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