You can use existing wires, but not always the same mounts. That's not a big deal. You can remove the mounts and seal the screw holes with a good sealant and replace the screw to keep the sealant in place.
The new mount may cover the original holes but make sure you seal them good!
If your wires don't match, you can always do wireless. Wireless works ok of you want to spend a little more for good signal broadcasting cameras that have a range to get to your dashboard. Most do. Dont buy it if the range isn't far enough for your dash to rear of the camper.
Wireless is awesome but be prepared to change batteries occasionally.