"Maintenance Free" batteries were introduced quite a few years ago and aren't in fact really maintenance free. To the best of my knowledge only the gel batteries (Optima as an example) are truly maintenance free. The batteries you have should more accurately be called low maintenance. I think the advance in design was really nothing more than a venting system that limits evaporation of electrolyte. Notice the key word "limit" and not "eliminate". You do need to check the level on these (that's why they make them so you can add fluid) just not as often as the older style. These are standard technology lead/acid batteries so adding distilled water should be safe. Don't overfill it and check it at least once a year. Also, if you have truly been running them with half the fluid gone then expect that they will likely fail fairly soon due to sulfation (usually results in a cell going bad and a voltage drop). Not much you can do about that at this point.

This all makes the old trick about the waterproof vs. water resistant watch come to mind.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold