I've never seen one over 28" and the price was ridiculous. If I was looking for a real HDTV (one over 49") I wouldn't even waste my time looking for a 12 VDC one. Get a 120 VAC TV and appropriately sized inverter.
Yeah you won't find many, even below 28" and the ones that are specifically designed to be 12V will be expensive (supply/demand--with cheap inverters available there really isn't much of a demand for 12V TVs).
The thing to do, however, is go to the store and look for TV's that have "bricks": A separate power supply and check the output of the brick. If its 12V you're good to go (the TV may not be listed as a 12V TV). Of course the brick outputs a regulated 12V (always 12V regardless of the input vs the 11-14V your RV's 12V rails can be--you'll need a 12V - 12V DC 2 DC converter for that). Probably a bit more effort than you're willing to do... Besides that; get above about 20" TV and they no longer have a brick.