First thing is to take a few pictures of your installation and post them. From that one of us can give you direction going forward.
Our 1990 Bounder had a plastic bezel around it and a box that held the old CRT model. I tool the bezel off and removed the part of the overhead that held the TV in place. I am a sawdust maker so I didn't worry about what to put in to hold the new TV, I just removed the old stuff first and then put something back to look somewhat original.
Another thing to remember is that the mounts that are RV specific are expensive but residential models are just as strong and CHEAP. I have one in the bedroom of our Winnie that I paid $15 for on line.
Patio door locks, velcro, straps, bungees all work to hold the TV in place.
If you are not a woodworker, I'd suggest you contact someone and let them build any cabinetry needed. The removal is pretty straight forward and if you are at all handy you should be able to do it.