Two comments
First water level... Battery instruction sheets often say to fill to the bottom of the "Split Ring" or the bottom of the filler tube (Same thing) Proper level however is 1/8 to 1/4 inch below that when filling.. Leave just a bit of "Wiggle room".. Refill with DISTILLED WATER when it's down near the top of the plates (But still over them) or when you feel it is needed whichever comes first.
Comment 2: Why are 6V better than 12 V
They are not.. at least not on paper... BUT what is different is this.
Six Volt are normally GC-2 Golf Car Batteries.. These are 220 give or take a bit Amp Hour, DEEP CYCLE batteries. they are NOT starting batteries.. Being DEEP CELL they are designed to deliver a relative slow discharge over a relative long time, and they have no problem with a state of charge as low as 50%.. Now.. Some may notice that in an emergency they WILL jump start your chassis.. This is true
But my chassis battery is 73 amp hours. so a 100 amp load is MORE than the C/20 rating, (A large load) for them, but less than half the C/20 rating of the GC-2's (A relatively small load)
Also, in the case of an Oh Cr**! level of discharge (Say 10 volts or less) fairly good chance of recovery if you don't do it too often.
Most 12 volt batteries sold for RV use are MARINE/deep cycle.. These are starting batteries with lip service to deep cycle,, they need to be kept fuller lile 60-70% minimum 50% SOC is an OH CR**! level on these batteries.. Still better than pure starting (add anothe 10%) and the chance of recovery from a real deep discharge is .. Well Slight.
Starting batteries the chance of recovery approaches ZERO.
Now why Six instead of say a 12 volt battery size 4D (Basically the same 220 amp hours but now at 12 volt)... Pick one up and one answer will pop out of your back.. Then go see your chiropractor. Six volts are 1/2 the voltage, 1/2 the WATTAGE (Same amp hour) and just slightly over 1/2 the POUNDS or if you like KGs. Which makes them 4 times easier to wrangle.
They are also the single most popular lead acid battery on earth, Golf cars from sea to sea and beyond in all directions use them.. This means the cost of re-tooling to make a different size is basically zero cause they never retool. and that means they cost less per unit.