I've installed several TV's in various RV's we have owned. Usually, I get a 20 inch LED HiDef television with built in DVD player. They are 12 volt (come with converter to run on 110 as well) and are available on E BAY in the $150 range. Cut out a piece of cardboard in the exact size of the TV screen you select. You can then determine where to install your mount so that the cardboard screen replica can swivel, extend or elevate/drop (depending on your mount) conveniently.
Remember, the ratio of screen size to your viewing distance may not require a large screen. Think of it something like this; a 40 inch screen from 10 feet away will appear to be about the same as a 20 inch screen from 5 feet away. In my current Class C, we use an 18 inch screen in the bedroom and its perfectly adequate.
Also, the smaller TVs have very tinny sounding speakers. Investigate upgrading to USB charged battery driven speakers you can plug into the "Headphone Out" jack on the TV. They'll operate 20+ hours on one charge.
Should you be unsure of where to locate framing to which you can attach your mount, EBay also has fibre optic endoscopes for around $12. They operate off your laptop USB port so you can peer inside your walls to determine where to drill for your TV mount.
Best of luck and I'm certain you'll enjoy having another conveniently located TV.