The water level should be just below the split ring which is about 1" or so below the top. Did you over fill?
Discharged batteries fail quicker. Since one's been replaced have the other battery checked at a battery shop or auto store, usually free.
Buy a digital voltmeter so that you can check the battery voltage and really know what is happening. Even the $10 Harbor Freight one should be OK. Read about battery charging voltage for bulk, absorb and float.
Your battery lights are often referred to as idiot lights. Their main purpose is for the salesman to point out what a wonderful monitor the RV has. What model battery charger do you have? Some are rather poor at charging batteries. A good 3 stage battery charger will charge a discharged battery at about 14.4V for bulk and then taper the amps for absorb and then lower the voltage to 13.4V for float.