Battery Water

Take the batteries into local auto parts/tire store and have them checked out (it's free) - if one of the batteries is bad then your converter is going to be working overtime to charge batteries which will result in water depletion.

How old are the batteries? Old batteries use more water. Lower voltage would help if they are old but healthy.

Here is a test. Disconnect one battery and see if it holds 12.65 volts for a few days. Then swap and check the other. Post the results.

What are your local temps? It is warming up here. Warmer temps plus a little too high float voltage from the converter (check and report voltage as requested above) will waste a lot of energy and use a lot of water. I am guessing the rig is plugged in at home?

Jim

Check the Batt voltage while charging, after the batteries are full charged.
If the voltage is over 13.4, your charger is not designed for maintaining, and will fume the water rapidly in warm weather.

Normal use causes the water to vent as fumes...that's why you need a ventilated battery box for non-sealed batteries. Do you have a 3-stage charger or are you just using the one that came with the RV? The stock equipment often keeps pushing a bulk charge to the batteries and can speed up the process. That's why folks recommend a battery tender when you're not using/maintaining the batteries often.

It's also why some folks use sealed batteries...almost no maintenance.