The whole point of the battery disconnect is to prevent what happened to you from occurring in the first place. While storing the camper, turn it to "off" so the battery won't go dead. Also, if you turn the battery disconnect off and still notice the converter is on (electronic looking thing, ours is under the queen bed), be sure to turn it off also. Ours stays on even when the battery disconnect is turned off so it's important to check or it will drain the battery down as well.
In the winter time, if you're in a cold climate, I would recommend bringing the battery indoors and keeping it charged. Although campers usually come with deep cycle batteries, a battery is happiest when it's near or fully charged. Full discharge can eventually damage the battery and reduce its overall capacity.