Hi acei,
8 years is possible with six volt jars. Capacity does initially increase (mostly in the first five cycles) and then slowly sinks into the sunset. If you are full time in the RV the life span may be shorter than the 8 year time frame. 500 cycles to 50% state of charge. At that point 80% of the capacity remains. With a larger amperage bank the number of cycles increases because the depth of discharge is smaller.
I'm one of the lucky ones who got 9.5 years from a pair of 12 volt marine batteries (I credit the 30 watt solar system on the previous RV for that).
