Also make sure you don't have a load on them. Sounds like something is draining them all the way down and they are staying discharged. I would put a battery disconnect in there and also keep a battery maintainer plugged into AC. They are cheap and use very little power but will keep the battery fully charged and will help you keep them for a long time.
If you don't want to put a battery disconnect in there, you can take off one of the battery cables.
Also use distilled water (only) in your battery. Tap water is not what you want.