Even for deep cycle batteries every time you allow them to discharge completely you will shorten their lives! As stated a trickle charge will keep the batteries topped off. To charge those house batteries will take at least 8 hours, charging them from the engine alternator in that state is at best "lame". severely discharged batteries require care and time to bring them back, (they still will have their life shortened). Keep them charged, do not over charge, and they will reward you with long life!