Try this. Pop the seat and disconnect the negative battery cable and isolate it. Properly charge the battery using a charger made for an AGM battery like the folks said above. When done charging, check the voltage. Come back an hour later and check the voltage again. Again, after another hour check the voltage again. If you see big drops like before, now that the negative cable is isolated, it really is the battery.
This battery, as well as the two previous ones were probably killed by the "tender". Get a real AGM charger like folks said above.
This one has been a favorite of mine now for a while and is available at WallyWorld.
http://www.batterychargers.com/spas3/
http://www.batterychargers.com/content/pdf/0099001561-R0.pdf