Even in the store position there is enough parasitic drain to run down the batteries. If you can plug in to 120 leave the switch in the use position, if you put it in store the batteries won't charge. Depending on your charger sometimes you can put it in start mode to deliver extra juice and get the batteries going again, just don't leave it that way very long.
If your charger still won't provide power you can use jumper cables from another vehicle for a while to get enough voltage so the charger will start.
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