There are three main types of lead-acid batteries:
- wet cell, or flooded
- gel cell
- absorbent glass mat (AGM)
Wet cell batteries
Wet cell batteries use a liquid electrolyte in an unsealed battery container. They're the cheapest type of battery, but require maintenance, and careful placement to ensure ventilation. While charging, these unsealed batteries will release hydrogen gas (highly explosive!) so must be installed in a well-vented location, outside of the main living quarters. If installed inside, they must be placed into a proper battery box with sufficient ventilation to the outside.
Gel cell batteries
Gel cell batteries use a thickening agent to keep the electrolyte from sloshing. They're a sealed type battery, meaning the electrolyte cannot be topped off with water if required (such as after overcharging), so careful attention must be paid to setting proper voltages. In addition, they're not able to accept as high a voltage as wet cells or AGMs, so make sure that your charger is calibrated for gel cell batteries.
Overall, gel cell batteries seem to exhibit the weak points of both wet cell batteries and AGM batteries (namely, finicky charging and high price, respectively) and should be avoided in favor of AGM.
Absorbent glass mat (AGM) batteries
The latest and greatest. These use a fiberglass mat inside the battery to hold the electrolyte in place. They can be installed on their sides, do not require extensive ventilation, can take a similar charge voltage to a flooded cell, and can accept huge amperage rates during charging. While a flooded cell battery might accept 1/10th of its amp-rating during charging (ie. a 100Ah wet cell can be charged at 10A), an AGM can take 5x or more (500A!). This significantly decreases the amount of time required to charge a battery (provided your charger can feed it that many amps!).
Additionally, AGMs self-discharge at about 1% of their charge per month, versus that amount per day for a wet cell battery.
Sizing your battery for your needs is very important - no amount of fancy charging equipment or expensive batteries can make up for a battery bank that's too small. On the other hand, there are disadvantages to oversizing your battery system (namely, weight and cost).
When figuring out the size of battery you require, keep in mind that batteries can only be discharged to about 50% of their capacity, before they get damaged. So a 220Ah battery can provide you with about 110Ah of power, before it's considered to be completely discharged.
6V vs 12V
There's a long-simmering and never-ending debate about the benefits of using 6V golf cart batteries, wired in series to achieve 12 volts.
The idea initially came about when true deep-cycle batteries were hard to come by. There weren't many available, and they were therefore expensive and difficult to replace. Golf cart batteries, on the other hand, were plentiful and available everywhere. They're designed to be run down to a low state of charge (deep-cycle) with their thick plates.
Nowadays, true deep-cycle batteries are widely available. While it's true that a good modern battery is more expensive than a golf cart battery, there's a reason - golf cart batteries are still unsealed batteries, requiring frequent maintenance and careful placement (for ventilation of explosive gases). The new AGM deep-cycle batteries are a much better choice, offering the same performance, with many additional benefits (sealed/non-venting, able to be installed on their sides, no maintenance required).
Skip the hassles of multiple 6V batteries, and find yourself a good AGM, deep-cycle 12V. Check out the recommended batteries section for some links.
There's no doubt that the best choice for a campervan is a 12V, AGM battery. My personal preference is for the Group 27 size of battery, which provides about 110Ah and weighs 65lbs. It costs a little more than half of what a 4D (220Ah) battery costs, but is much easier to move and to install, at half the size and weight.
As far as specific brands:
Lifeline are the premium choice right now. They're built very well and have a good warranty.
Universal are built by Lifeline. There are some internal differences, I believe, but they are also considered to be a high-quality battery, for a bit less money.
Comparing Marine Battery Technologies, at vonwentzel.net. An exhaustive investigation into battery types, charging methods, price comparisons, and more, plus a number of additional links.