The two battery types are constructed differently for specific purposes.
A general purpose (starting) battery provides short power bursts to engage the engine. It can be quickly destroyed if subject to deep drawdown/recharge cycles.
The deep discharge is specifically intended for frequent deep discharge/recharge cycles. It will last much longer in the RV environment where purpose is to get as much 12v for as long as possible and then recharge.
You want both RV batteries to be deep discharge type, either 12v in parallel or 6v in series. Sorry I don't know why 6v is superior, and its near my bedtime.
