There should be a backer board in the area the tv is to be mounted. Our unit has wall switches and outlets below. I removed the low voltage outlets, used a mirror and a flashlight to see up between the wall panels. When I located the backer board, I took a tape measure, ran it up inside the wall until it made contact with the bottom of the backer board. I took the measure to the top of the low voltage opening and recorded it. Then on the out side of the wall, I measured up and put a piece of masking tape to locate the bottom of the backer board on the outside of the wall. Then from the ceiling, I measured down to locate the bottom of the backer at any point. Anything above those lines would be strong enough to hold the mount.
Once I located the mounting location, I carefully located a hole to mount a low voltage aftermarket wall box. This box is actually only a frame that takes standard rectangular wall plates. Our TV set is 12 volts, so I connected 12 volts from the back side of the outlet I had removed and ran a 12 volt circuit through the wall to the TV. I did the same thing with the TV coax cable wire. Clean install, no wires showing.
This takes time and patience and a little McGiver, but it is worth the effort.
Just make sure that any cuts, screws, etc. do not interfere with the existing wiring/plumbing. Any small mirror will work. I used DW's compact mirror. I found that removing existing wall plates made the job easy. If you are working with AC circuits, make certain that they are off.