It depends what you are using. If storing the coach with no load (everything off) I find that the batteries will last about a week or 10 days before the get below 12v. If I open the breaker in the battery box or you have a manual disconnect installed, the batteries will last several weeks or longer. With a small solar panel hooked up directly to the batteries and when they are disconnected, they will last indefinitely.
The chassis battery also drains naturally, but it seems to last longer. However, draining the chassis battery below 11.5v will ruin it much more quickly than doing that to your coach batteries. The coach batteries are designed to deep discharge (although that also shortens their life). The chassis batteries are not designed for deep discharge.
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