A 2x4 mounted vertically in the existing cabinet makes a great support "beam". If you use an articulated mount you can get the TV to swing out toward the mid-line of the coach.
I've just replaced our existing articulated mount with one that both swings and lowers the TV so when it is being watched it is well below the level of the upper cabinets and just about in the middle of the coach. That puts it into an excellent viewing position both for someone on the sofa and another person on the recliner. My DW's complaint about our previous mount was that the TV was "too high" and her neck hurt from having to look up at it; this way it is nearly at head height.