If the generator started readily, then they most likely still have a sufficient charge and may not need replacing. If you have a multi meter, test them to make sure you have enough voltage. I would not replace them just yet, unless they are getting past five years old. You said you left it "plugged in," so I will assume that the converter kept the batteries maintained. With the engine running, and the brake set, you should be able to operate the slide to allow you to check the fuse box. Most likely, something tripped, and is most likely the culprit, instead of the batteries.