Read This First: Disclaimer: I've written what I personally did, and my opinions. Don't assume what I did was safe, and don't assume it will work for you. Do more research, and make your own choices. I am not responsible for your outcomes! :-)
There are several different types of deep cycle RV batteries available. Much information is on the web, so I don't intend to duplicate it all here. I'll give you a general overview of battery selection, talk a little about my own choices, and give you lots of links to sites with much much more detail.
Basic Battery Types
First, we're NOT talking about Automotive Starting Batteries. These are made to provide a lot of power quickly for a short time to start your engine. You may have one or two of these for starting your RV engine, and that's where they belong. We're also not talking about most "Marine" batteries like you'd find at Wal-Mart. These typically are a compromise between starting batteries, and true deep cycle batteries.
What you need for an RV "Coach" or "House" battery is a true deep-cycle battery. These are designed to store a large quantity of power, to discharge deeply, and to recharge over and over again. There are three basic types of these.
Lead Acid Flooded Cell batteries are the typical batteries that you see in most deep cycle applications. They are available in several sizes, in both 12v and 6v. They have very thick lead plates submerged in an acid solution (electrolyte). The majority of RV's use this type of battery.
Many RV's have two or more 6 volt batteries, wired in series and parallel to create a 12 volt battery bank. Golf Cart batteries are the primary example, and Trojan brand size T-105 is the industry standard.
Gel Cell Batteries are similar to Flooded Cell batteries, except the electrolyte is gelled. For this reason they require little maintenance and rarely if ever emit gasses.
AGM (Absorbed Glass Mat) batteries have a glass mat material between the plates and the electrolyte is absorbed into this glass mat. This makes them "spill proof" and they're the only battery that can be shipped via UPS or FedEx. Being spill proof also gives them advantages in RV and marine use.
What do I have? I bought a set of AGM's for my Christmas present in 2004. I installed them in an un-vented forward compartment under our motor home. They're the most high-end item in our otherwise frugal 12v electrical system. See link below.
**NOTE We have our AGM's installed in an un-vented compartment. While this is generally accepted as safe, I'm told by experts in construction that it's technically against the National Electrical Code (no exception for AGMs). I feel safe with our setup, but you need to research this topic and come to your own conclusions.
6 volt, 12 volt, how's that work???
Easier than you think, here are the rules:
Two 6 Volt, 200 AH batteries in series, gives you 12 Volts, and 200 AH
Put these in parallel with another two 6 Volt batteries, and you have 12 Volts, and 400 AH
(From the chapter "Intro to Dry Camping". Scroll down and skip this if you've already read it.
After two+ years of dry camping, and watching others do it, I've come to the conclusion that the lack of an adequate battery bank is the #1 deterrent to successful (i.e. frugal, fun, and quiet) dry camping. Your batteries are the heart of your system, and if they are inadequate, or if you're not fully utilizing them, you'll be a slave to your generator and the bane of neighbors who would like to enjoy a little quiet from time to time.
Without the ability to store sufficient power to meet your daily needs you're going to have to run your generator most of the time, and that's costly, noisy, and totally unnecessary. We've seen people who run a generator 8 to 12 and even 16 hours a day. There's just no reason for that. With an adequate battery bank, a good inverter/charger, and a little conservation, you should be able to meet all your energy needs by running the generator just a few hours a day, even without solar power.
Your situation will vary, but here are the basics for a non-solar RV:
Here are some links for more detailed reading:
Our Batteries - Here's our little 4 battery bank
12 Volt Side Of Live - Part 1 - Excellent article by Mark Nemeth on Batteries and Chargers
General FAQ about Batteries - from Arizona Wind & Sun's web site
Phred's Poop Sheet on Batteries - Good basic to intermediate primer on battery power
Trojan Batteries Home Page - The industry leader for Lead Acid batteries
Concorde Lifeline RV Batteries - The "Cadillac" of RV and Marine batteries using AGM technology