I, on the other hand, have left batteries attached to my battery tender for MONTHS and not had a single problem
If we're talking actual Battery Tender brand chargers, they drop into "trickle charge mode" once the battery is full. Mine has a separate light to indicate when it is in this mode.
If we're talking about a dumb unregulated battery charger, then yes they can boil off the electrolyte in the battery, but the OP stated that fluid levels were good.
If the batter was putting out ZERO voltage, then there's a physical problem inside the battery, like a broken connection to the posts or between plates. Even a very old nearly dry battery will put out some power just not enough to do useful work with.
So unless you Battery Tender is broken, it shouldn't be doing this to batteries. It is possible that yours is broken and over charging, but you'd need to hook up an ammeter to tell this.
mystic_1