Good news that they are still usable. ( I did say they weren't all the way dead ISTR) I don't know what is normal for water topping up timing though.
My 6s need a little water every month or two. I can't tell if it is from Floating at perhaps too high a voltage for the ambient temperature (doubt that), or from frequent shorter term high amp dicharging and charging when being deep cycled.
I am amazed to read on here about people who hardly ever have to add water to their batteries. I wonder sometimes if that is a sign that I am a bad person! :)
In any case, I find it is vital to check the levels in the batteries at least once a month and top up as required. Also then to do the Equalize trick with the VEC1093DBD. These are fairly new 6s too.
I just use the hydrometer to suck up fresh water and squirt it in the cells. I don't know how much water that comes to in ounces or cubic pixocaples or whatever we are supposed to be using now as a unit of water. :(
It seems the problem with the OP's batteries is that they are so hard to get at to monitor them. Is it possible to tow a little cargo trailer that has the battery bank in it and the big inverter, with a solar panel on top? That would help for ease of access and be sort of a nifty project for the handy-man too. ISTR at least one other Forum member does this or similar.
