There are many web sites that explain the various types of lead-acid deep cycle batteries available for RVs ( batteryfaq.org) ( Comparing Marine Battery Technologies) ( Batteries and other Electrical Stuff). The bottom line is that Absorbed Glass Mat (AGM) batteries are the latest and greatest technology. These batteries are completely sealed and, in normal situations, emit no hydrogen or other gases. Because of this, these batteries can be mounted pretty much anywhere inside or outside an RV and in any position (other than upside down). The battery acid is bound within the glass mat, which is sandwiched between the lead plates. There is no free liquid or gel within these batteries to worry about. The best technical manual on AGM batteries can be found here ( Sealed, Gel⁄AGM Technical Manual ). This is put out by East Penn Manufacturing Company, who is one of the largest battery manufacturers in the USA. I ended up buying two AGM batteries at a local battery store for $150 each. These were made by East Penn and are called Deka Sea Mate. For the Roadtrek, you are limited to the Group 27 battery size. The Deka batteries each have a capacity of 92 amp-hours. Since you should never discharge a battery more than 50% if you want to prolong battery life (less is even better), the two batteries connected in parallel will provide up to 92 amp-hours of usable power before you need to recharge them. I couldn't get any bigger ones, as these are even a tight fit in each of the two outside battery compartments on my Roadtrek (one in front of the right rear wheel, the other behind it). I had considered moving the batteries inside, particularly to keep away from the exhaust heat of the tail pipe, which runs directly under the rear battery compartment. In the end, I felt I'd lose too much storage space so decided to keep the batteries where they were designed to reside.