As the previous guys said. With a car the ground path is just about everywhere. With your RV most of your lights and stuff use a separate wire that runs back to the ground. As they said. Connect your meter or a test light to the positive wire at the light and run a test piece of wire to the frame or battery negative. Do the same with the negative hooked to your test light and jumper it back to a good positive or battery positive. You need to prove which side of the circuit you have lost. Its unlikely that you need to run new wire outside etc. You probably have a very easy fix at one of the fuse, or connections.