Some vehicles, by design have a parasitic electrical drain, in some cases a computer that runs diagnostics even when the ignition is off. One of my cars came with a paragraph in the manual that says, never leave parked for more than 28 days, as the battery will be drained enough to not allow it to start. My former DP would drain both chassis batteries in two weeks, until I installed a small intelligent charger connected to the chassis batteries, that would power up when ever the generator ran or it was plugged into shore power.
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