Somebody here will tell you the group size originally specified, but basically you can use anything that will fit the opening.
If by "posts" you mean caps, what you have is a 12 volt
battery - similar to a car battery. In fact, that may be what you have. You have housings for two, apparently, but you actually only need one to use the coach. Two are nice for boondocking.
There are a lot of choices in batteries, and the price range is quite wide. If you want two, you can connect two six volts in series to get the 12 volts you need, or two 12 volts in parallel.
Take a look at http://www.phrannie.org/phredex.html
for a lot of good information, including batteries.