My experience is that the batteries can run down in a few weeks even with the battery switch set to store. If this happens enough it will kill the batteries prematurely. That is why I have new batteries in a one year old trailer.
Now, if I'm going to store the trailer for more than a few days, I physically disconnect the positive cables from the batteries. If it's going to 3 weeks or longer, I pull the batteries and take them home so I can use a battery tender to keep them charged up.
You should use a digital multimeter to se what voltage is left in your batteries. If they are below 11.58 volts, they could be toast.