An email response from Interstate said that the life of a deep cycle battery will depend upon how much discharge occurs in each cycle.
100% deep cycle = approx. 300+ cycles
50% deep cycle = approx. 1000+ cycles
25% deep cycle = approx. 2200+ cycles
So, unlike cell phone batteries that do best with nearly complete discharging, deep cycle golf cart batteries will last longer with less discharge - over three times longer when dropping to 50% vs 100 percent.
Interstate also reinforced the idea that sitting for any period of time completely discharged is very hard on a battery.
It is also possible that our batteries were prematurely aged when we purchased the RV. The dealer was located in an area that has very hot summers, with many days over 100°. If the RV dealer did not keep the charge up in that hot weather, the batteries were doomed to begin with.