RV Batteries?
I will assume that the chassis battery is still connected? If so then all that you need do is to seperate the leads remaining by where they go. All leads that supply any power must be connected on the + side of the battery and there should be one or two ground leads that go to the - side. If you choose to go with two 6V batteries, they need to be connected in series to work as one large battery. That means that the + of one connects to the - or the other, with a short, heavy battery cable. The remaining two posts become the + and - of one very large battery with the - connected to ground and all power leads to the other.
To find what leads go to +(positive) and which go to -(neg.) all that you need do is to use a good ohm meter and read from each one to the chassis. The ground side leads should read no more than one or two ohms and even with a poor connection it will be a single digit number. You can also trace those leads back to make sure. Ground leads are also usually black or green. The hot leads for 12V systems are usually red or white. But since this is a used RV it may not be true. Traceing the wires is the only way to be completely sure if any lead has a resistance value that is low, but more than one or two ohms.