There are two ways to wire in dual batteries on a motorhome or camper. One way is to wire the batteries in parallel, which does not change the voltage of the joined batteries. Two parallel-wired 12-volt batteries will provide more power, but the voltage remains 12 volts, unlike two 6-volt batteries wired in a series. In a serial connection, the two 6-volt batteries yield 12 volts. Output voltage in a serial connection is the sum of all voltage of the batteries within the connection. A typical tube flashlight stacks two 1.5-volt batteries to light a 3-volt light bulb.
Spot two 12-volt batteries next to each other that will fit in the battery box area of the camper or motorhome.
Connect the positive and negative terminals of one battery to the positive and negative cables of the camper. Connect two cables --- one to the positive terminal and one to the negative terminal of the unconnected battery. Both these cables should have eye connectors on the other end.
Bolt the positive cable eye of the unconnected battery to the positive terminal of the battery that's connected to the camper. Bolt the negative cable eye of the unconnected battery to the negative terminal of the battery that's connected to the camper.
Spot two 6-volt batteries side-by-side that will fit in the battery area of the camper. Two 6-volt batteries in series will yield a 12-volt output.
Connect the positive terminal of one battery to the negative terminal of the other battery. Use a battery cable with one positive and one negative terminal to make the connection.
Connect the two open posts of the two batteries in the serial connection to the camper. Connect the open positive post of one battery to the positive cable of the camper and the open negative post on the other battery to the negative cable of the camper. The serial connection will yield 12 volts at the camper connection.
The parallel battery cables should have a battery terminal on one end with an eye loop on the other end. Positive-to-positive and negative-to-negative is a parallel connection. If the batteries are 12 volt, the parallel connections remain 12 volt.
Always wear eye protection when working on or near lead acid batteries. Lead acid batteries can explode if a spark ignites the acid.