So it turns out many RVs (and for sure our Sprinter) have pretty massive “vampire” loads. If you park my van, even for a fairly short period (say 10 days) you could come back to a battery that’s depleted.
That’s because the house charger will not charge the chassis battery from AC, and the chassis has small drains that add up over weeks.
You can buy a device that does charge your chassis battery from the house.
Or you can do as some people do and pull the master disconnect chassis switch on the floor. That has its drawbacks though. You are supposed to remember to wait a certain period of time to let the computer do its thing before pulling the switch.
And your AGM battery may still go dead in a few months.
So for us this was a much better solution. Just plug a small battery tender in the lower cigarette lighter socket that’s always on, and no more worries.
I’ve been using battery tenders for years, once with a motorcycle for almost a decade, and the multi stage charger really babies the battery.
We turn this on whenever we aren’t returning for more than a couple days. Simple, and it doesn’t matter whether you return the next day or 4 months later.