Disconnect the power source of the camper to avoid any possibility of electric shock.
Use a screwdriver to remove the housing around your ceiling bulb.
Remove the bulb from the light socket. Instead of a standard household screw-in bulb, the12 volt bulb probably has 2 prongs that allow it to "plug" in to the outlet. Locate the socket and pull the bulb away from it to remove it.
Place your new bulb in the socket.
Reconnect the power to your camper and test out the new bulb.
Reinstall the plastic housing around your bulb once you are sure the bulb is working properly.