My parents were told the same thing about their batteries. DH is saying that yes, they will go flat slowly, like a trickle. They will discharge over time if not in use. And like a car battery, if they go completely flat, then they're pretty much stuffed. A lot depends on the type of battery - deep cycle batteries are different.

Personally, I don't know much about all of this! I asked him your question and am writing out his answer. He said if it was our van, he'd put in an isolator switch where he could switch them off and then charge them up once a month or so.

If it was me, I'd go with the manufacturer's advice simply because they will be the one to cover the warranty if the batteries don't last the way they are supposed to