Start by drilling holes in the back of your fan housing. The size will depend on the size of the cable glands. Mine was a 5/8 inch hole, which then had to be widened just slightly with some sanding. I almost goofed and made the hole too high before lowering the cover to the fan and seeing that it would hit the cable gland.
Back inside, with the cables passing through the glands, you’ll need to remove the cover to the fan and drill a large enough hole for the cables to exit the fan area. My electrical will be in the galley so my cables will go straight back and through a hole in the bulkhead with other wires.
Here are the cables as they exit the fan housing outside. You need to pay attention to what type of connector is on each so they match up with your panel connectors. These cables are about three feet long and the other ends will be in the galley. Connected to my solar charge controller are longer cables (about 10 feet) that can connect to those ends when the solar panel is on top of the camper. If I ever want to place the panel in a better spot, I disconnect it from these shorter ones and connect directly to the longer ones going to the charge controller.
Here’s a view of the solar panel connection. As the guy points out in his video, the holes for the cables are protected from rain by the fan cover and the solar panel (when it’s installed). I like that there are no holes going through the wood of the camper itself and you’re basically using a hole that was already there. I didn’t show it but I did have to modify (cut) some of the inside plastic of the fan to allow the cables bending room.