I mounted a bracket on the back wall, 03 Dolphin LX 6355 there was a metal beam running across the top of the window a header of sorts and extending the full width of the back wall. It accepted large metal screws (lags) just fine. A I recall it must be steel because I think I used a magnet to verify its existence. I used it to a fix a 3/4 thick square board to then accept the base of the mount giving me more room for heavy screws for the mount. The TV back there was not heavy at all and it rides just fine, mine articulates back into a cabinet where the original CRT sat before.
I do all my own stunts
03 Dolphin LX 6355, Workhorse W22, 8.1 vortec, 04 CR-V, Blue Ox, Brake Pro----Norm, Barb and
Doc(He's a PhD)