Did I kill my deep cycle battery ? Also, how to avoid this.

Went to Fraser over Easter with my brand new van and Delkor 100ah deep cycle battery. Ran the Dometic 3 way fridge on gas while camped, and switched to 12v for the long trip back to Brisbane- via Anderson plug to car alternator and starting battery. When I got back, I discovered that the Anderson plug mustn't have made connection (maybe it was the sand in it from driving around the beach for the week), and the deep cycle had been running the fridge for the whole 7 hour trip back(after running the lights and water pump for the week). It was very low. The smart charger wouldn't charge it (it refuses to charge if the volts are lower than 9 volts- according to the book, if less than 9 volts, then the battery has had it). I "fooled" the charger by connecting up an old charger briefly to show 12 volts, and away the smart charger went. After 3 days, it showed "fully charged", and dropped back to a trickle. According to the charger, it is full and OK.
There is a hydrometer spot thingy on the battery which shows green for "charged", black for "needs charging" and clear for "battery is stuffed". It showed green when we went away, but now only shows black. It has never showed clear.
My question, did I just use up a lot of this battery's life ? Does anyone know anything about those spot hydrometer things ? Any hints on how to avoid this in future ? I just can't tell if a connection to the Anderson plug has been made OK.

Would appreciate your thoughts.