I don't have the model camper you do, but I'm guessing the stud finder isn't finding the TV mount backer because it isn't dense enough, it's usually just a piece of plywood, or similar material.
I would contact Keystone directly and ask them where the backer SHOULD be. Ask for measurements from easily identified reference points like the ceiling and the left edge of the bathroom door to the center of the backer.
Their contact page can be found at https://www.keystonerv.com/owners/contact-us/
Phone may be faster, but email gives you a written record of your inquiry and their response.