Turn the RV off and place it in park.
Set the second battery in the battery box next to the first battery. Turn the second battery so that polarity posts are on the same side as the first battery. For example, the positive posts of both batteries need to be on either the left or right side of the battery box.
Place a screwdriver on the positive terminal of the first battery. Turn the screwdriver counterclockwise to loosen the terminal. Pull the terminal straight up and off the battery post. Repeat this process to remove the negative terminal on the first battery.
Remove the positive and negative terminals off the second battery in the same manner.
Place each end of a white battery cable over the positive post on the first and second batteries. Replace the positive terminals on both batteries by setting them on the posts and tightening the screw in a clockwise direction.
Place each end of a white battery cable on each negative post and tighten them in the same manner to complete the parallel wiring circuit on both batteries.
