It depends on how the camera was wired.
If hooked to the clearance lights, your tow vehicle must be hooked up and the lights turned on.
If hooked directly to the RV batteries, you should be able to see a little blue light on the camera.
If it is hooked directly to the batteries, whoever installed it, may not had installed a fuse in the power lead.
Being used RV, when was the camera installed? Was it installed when purchased new, or did you have it installed?
After you figure out how the power gets to the camera, plug the camera into the tow vehicle and turn on.