I removed the ~90 pound flat screen tube TV in my 2007 Seabreeze with about 10 pounds of up-to-date flat screen and lumber. I fastened a piece of 3/4" plywood to 2x4s that I mounted to the existing steel enclosure. Then I hung the new TV on a mount that latches closed. There was a gap at the bottom that I covered with another piece of plywood that is held in place with a strong magnet. Here are a few pictures:
After removing the tube TV. There is a very robust steel enclosure that held the old TV.
The 2x4 on the bottom is attached through holes drilled in the bottom of the steel enclosure. The top 2x4 is attached with L brackets to the top sides using the screw holes that attach the enclosure to the cabinets.
Here you can see the mount attached to the plywood that is attached to the 2x4s. It has been painted flat black. The bracket latches closed and I put some weather stripping on the edges of the TV to hold everything secure from road vibrations. (I don't know why the picture is sideways.)
The finished product.