This stuff is available on the web;
The 3 main types of deep cycle batteries are wet cell, gel cell and AGM.
FACT - If you use a wet or gel cell until it is flat, kiss it goodbye.
FACT - If you use an AGM to empty a small number of times, it will recover on charge, to a reasonable extent, but its life will be drastically reduced.
This next bit applies to all 3 types, but the AGM will last longer;
FACT - When my caravan is in the driveway, it is on power all the time so that the smart charger is keeping the battery topped up. If I turn on the LED lights or other devices that the total current draw is less than what the charger is putting out, then I am NOT cycling the battery.
FACT - A deep cycle battery's life is governed by how many times it is cycled. So assume it's night time, no solar input and assume you are out bush and the genny is off. If you use power from the battery until the next day before the sun operates the solar or you start the genny, then that is a cycle. The following rule of thumb applies to how much is used vs how many cycles you will get. I'll use a 100AH as the example, but the % applies regardless of the battery or ganged battery total AH.
0 draw - on charge = no cycle
0 draw - not on charge = a slow discharge over time and is not good
10 to 20% draw - not charging = a cycle life of 1,000 approx.
50% draw - not charging = a cycle life of 300 approx.
5% approx. draw - not charging = there is a low cycle detrimental effect on deep cycle batteries that can shorten a battery life to the same extent as a 50% draw. So if you are using power, make sure you use more than too little before charging.
> 50% draw - no charge = My advice would be to buy some shares in a battery dealership, because you'll be buying a lot of batteries.
The obvious conclusion is, that the most economical battery system needs monitoring properly. You need to know what you are using and what and when you recharge. ie an Ammeter.
ie this kind of device below, I use one in the 4x4 and another in the van.
http://www.jaycar.com.au/productView.asp?ID=MS6170&CATID=83&form=CAT2&SUBCATID=1003#3