i'm a big fan of 6volt deep cycle batteries. The ones meant either for golf carts, or electric fork trucks. Connect pairs of them in series to make 12 volts. These batteries are not cheap, but are the best in lead acid batteries when it comes to running accessories in your skoolie when the engine is not runing.
If you use automotive batteries, or wal-mart marine/deep cycle/RV batteries, you're getting what you pay for. These types of batteries are not meant to run inverters and other loads for long periods of time or to be drawn down below 12.3 volts resting voltage (12 volt batteries are actually ~12.6 volts when fully charged. You should never let batteries drop below 12.3 volts resting voltage!) Sure you can make it work, but as far as lead acid batts go, you can beat the performance you get from deep cycles.
Trojan makes some of the best deep cycle batteries. http://www.trojan-battery.com/GolfDC.asp?Product=51
IF you plan to keep your skoolie for 5 or 10 years, it's worth it to make the investment in good batteries. In the long run, you'll spend more money replacing cheap batteries than you would have if you purchased the correct batteries in the begining.
for all kinds of info on batteres, go to http://www.phrannie.org/battery.html
Next thing to consider: How to charge them properly............
In my opinion, creating a 24 volt "house" system doesn't make good sense if you're bus is already set up for 12 volts. In theory, 24 volts is a better idea than 12. Current can be carried in smaller guage wires, motors run more efficiently ect.......however, there is a problem: we currently live in a 12 volt world. Almost all accessories commonly available today for any vehicle in north america are for 12 volts. Fuses, lights, fans, motors, inverteres, relays, phone chargers,coolers, refers, ect are almost exclusively 12 volts.