If you will not need batteries for power, depending on your converter, you may or may not still need a 12v
battery supply to have the converter convert. Sounds like you may get away with any 12v
battery and probably only one. Ideally you would want a deep cycle battery rather than a starter battery for auto use. But this also may not be necessary for use as you describe. Plugging into 110 AC without any battery may be possible, but also may not be a good idea for your converter.