How do I mount a TV on a solid wood wall?
I agree with BMitch that there's horizontal strapping over the studs. You might want to try a Bosch GMS120 Wall Scanner instead of a regular stud finder. It claims it can find wood substructures 1-1/2" deep which is going to be about how deep your studs are from the front of the panels.

The other side of the wall is probably an exterior wall which will make this difficult but try finding the studs from that side. Also electrical boxes are usually mounted on the side of studs so use them as a cue.

Another option, would be to try and figure out where the studs are through trial and error. Measure out from one corner, factor in the depth of the panels and strapping from the side wall then over your fireplace drill a small exploratory hole near where a stud should be but not directly over the strapping. Poke around with a wire bent into the shape of an L and see if you can find a stud. If you can't try measuring from the opposite wall. Once you find one you should be able to easily find the rest since they should be space 16" on center.

Just remember to get longer lag bolts.

Also, there are some replies in this forum thread that might be useful to you.

I had to mount a TV fairly high. Make sure you get a mount that has tilt control to reduce neck strain.