You really need to try and understand your system and how it works. Take a look at this
12 Volt Side of Life This will help you understand your system.
Most all RV's have some kind of gauge panel for their tanks. On that should be a battery fullness indicator. What does it say when you are getting the alarm beeps?
I agree with RJsfishin that your converter which by the way is what supplies DC voltage to your lights and all things DC and charges your battery, is not working. If you have a standalone automotive battery charger, put it on your batteries for several hours. If you don't have one you should consider getting one (20-40 AMPS Smart or 3 stage). A lot of RV'ers carry one as a back up to their converter which is really just a battery charger.
